Hi Graeme,

Is the dropdown box a combobox or data validation?

For data validation try this, although if it's a combobox it might be better
to load it every time the workbook is opened with the options being dependant
on who is logged onto the computer. That way if they disable macros they
won't be able to select the c/d without invoking the password.

Public myval 'value selected
Public hasRun As Boolean 'to prevent the inputbox recurrng

Private Sub Worksheet_Change(ByVal Target As Range)
 If hasRun = False Then
   Select Case Range("B4").Text
     Case "C", "D"
       myval = Range("B4") 'remember what was chosen
       hasRun = True
       Range("B4") = ""  'remove the contents of B4
       If Application.InputBox("enter password") = "password" Then
         Range("B4") = myval  'restore the chosen value
       Else
         Range("B4") = "" 'leave blank
       End If
       hasRun = False
   End Select
 End If
End Sub