Use the CellRadioGroup property to add or remove a radio button from a group. In a radio group only one radio button can be checked. A radio cell cannot be contained by two different radio groups. Use the CellHasRadioButton property to add a radio button to a cell. When the cell's state is changed the control fires the CellStateChanged event. The CellState property specifies the cell's state. By default, when a cell of radio type is created  the radio cell is not grouped to any of existent radio groups. 

The following VB sample shows how to set the radio type for all cells of the first column, and groups all of them in the same radio group ( 1234 ):

Dim h As Variant
ComboBox1.BeginUpdate
With ComboBox1.Items
For Each h In ComboBox1.Items
    .CellHasRadioButton(h, 0) = True
    .CellRadioGroup(h, 0) = 1234
Next
End With
ComboBox1.EndUpdate

or

Private Sub ComboBox1_InsertItem(ByVal Item As EXCOMBOBOXLibCtl.HITEM)
    With ComboBox1.Items
        .CellHasRadioButton(Item, 0) = True
        .CellRadioGroup(Item, 0) = 1234
    End With
End Sub

To find out the radio cell that is checked in the radio group 1234 you can use: MsgBox ComboBox1.Items.CellCaption(, ComboBox1.Items.CellChecked(1234)) 

The following VB sample groups all cells of the first column into a radio group, and displays the checked cell:

Private Sub ComboBox1_InsertItem(ByVal Item As EXCOMBOBOXLibCtl.HITEM)
    With ComboBox1.Items
        .CellHasRadioButton(Item, 0) = True
        .CellRadioGroup(Item, 0) = 1234
    End With
End Sub

Private Sub ComboBox1_CellStateChanged(ByVal Cell As EXCOMBOBOXLibCtl.HCELL)
    With ComboBox1.Items
        Debug.Print "In the 1234 radio group the """ & .CellCaption(, .CellChecked(1234)) & """ is checked."
    End With
End Sub

The following VB sample assigns a radio button to the focused cell:

With ComboBox1.Items
    .CellHasRadioButton(.FocusItem, 0) = True
    .CellRadioGroup(.FocusItem, 0) = 1234
End With

The following C++ sample assigns a radio button to the focused cell:

#include "Items.h"
CItems items = m_combobox.GetItems();
items.SetCellHasRadioButton( COleVariant( items.GetFocusItem() ), COleVariant( (long)0 ), TRUE );
items.SetCellRadioGroup( COleVariant( items.GetFocusItem() ), COleVariant( (long)0 ), 1234 );

The following VB.NET sample assigns a radio button to the focused cell:

With AxComboBox1.Items
    .CellHasRadioButton(.FocusItem, 0) = True
    .CellRadioGroup(.FocusItem, 0) = 1234
End With

The following C# sample assigns a radio button to the focused cell:

axComboBox1.Items.set_CellHasRadioButton(axComboBox1.Items.FocusItem, 0, true);
axComboBox1.Items.set_CellRadioGroup(axComboBox1.Items.FocusItem, 0, 1234);

The following VFP sample assigns a radio button to the focused cell:

with thisform.ComboBox1.Items
	.DefaultItem = .FocusItem
	.CellHasRadioButton(0,0) = .t.
	.CellRadioGroup(0,0) = 1234
endwith

Note: The intersection of an item with a column defines a cell. Each cell is uniquely represented by its handle. The cell's handle is of HCELL type, that's equivalent with a long type. All properties of Items object that have two parameters Item and ColIndex, refer a cell.

The following lines are equivalents and each of them changes the bold font attribute of the first cell on the first item.

With ComboBox1
    .Items.CellBold(, .Items.ItemCell(.Items(0), 0)) = True
    .Items.CellBold(.Items(0), 0) = True
    .Items.CellBold(.Items(0)) = True
    .Items.CellBold(.Items.ItemByIndex(0)) = True
    .Items.CellBold(.Items.ItemByIndex(0), 0) = True
    .Items.CellBold(.Items(0), ComboBox1.Columns(0).Caption) = True
End With