<font face=Courier New><SPAN style="color:#00007F">Private</SPAN> <SPAN style="color:#00007F">Sub</SPAN> Worksheet_Change(<SPAN style="color:#00007F">ByVal</SPAN> Target <SPAN style="color:#00007F">As</SPAN> Range)
Application.EnableEvents = <SPAN style="color:#00007F">False</SPAN>
<SPAN style="color:#00007F">If</SPAN> <SPAN style="color:#00007F">Not</SPAN> Intersect(Target, [A1]) <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> <SPAN style="color:#00007F">Then</SPAN>
[B1] = [B1] + [A1]
<SPAN style="color:#00007F">ElseIf</SPAN> <SPAN style="color:#00007F">Not</SPAN> Intersect(Target, [B1]) <SPAN style="color:#00007F">Is</SPAN> <SPAN style="color:#00007F">Nothing</SPAN> <SPAN style="color:#00007F">Then</SPAN>
MsgBox "Tsk, Tsk... Keepa you hands off!"
Application.Undo
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">If</SPAN>
Application.EnableEvents = <SPAN style="color:#00007F">True</SPAN>
<SPAN style="color:#00007F">End</SPAN> <SPAN style="color:#00007F">Sub</SPAN>
</FONT>
You can use event code such as the above. When are you planning to reset B1?
Y'know, there used to be a gadget called a calculator...