Hello,
replace your code by this:
<div style="background-color:#FFFFFF; border-width:2px; border-style: groove; border-color:#ff9966; padding:4px;"><nobr><span style="font-family:Courier New,Arial; font-size:9pt ;" ><span style="color:#000080"; >With</span> Selection.Validation
.Delete
.Add Type:=xlValidateList, AlertStyle:=xlValidAlertWarning, Operator:=xlBetween, Formula1:=<span style="color:#800000"; >"=$HA$1:$HM$1"</span>
.IgnoreBlank = <span style="color:#000080"; >True</span>
.InCellDropdown = <span style="color:#000080"; >True</span>
.InputTitle = ""
.ErrorTitle = ""
.InputMessage = ""
.ErrorMessage = ""
.ShowInput = <span style="color:#000080"; >False</span>
.ShowError = <span style="color:#000080"; >False</span>
<span style="color:#000080"; >End</span> <span style="color:#000080"; >With</span>
</span></nobr></div>
Because of the length of your string in the warning cell (HM2), I would recomment you to use a msg-box instead of the validation input-msg.
Your cell with the dropdown would be A1.
Then copy following code into the codewindow of the sheet in question:
<div style="background-color:#FFFFFF; border-width:2px; border-style: groove; border-color:#ff9966; padding:4px;"><nobr><span style="font-family:Courier New,Arial; font-size:9pt ;" ><span style="color:#000080"; >Private</span> <span style="color:#000080"; >Sub</span> Worksheet_Change(<span style="color:#000080"; >ByVal</span> Target <span style="color:#000080"; >As</span> Range)
<span style="color:#000080"; >If</span> Target.Address = <span style="color:#800000"; >"$A$1"</span> <span style="color:#000080"; >Then</span>
<span style="color:#000080"; >If</span> [COUNTIF(HA1:HM1,A1)] = 0 <span style="color:#000080"; >Then</span>
Application.EnableEvents = <span style="color:#000080"; >False</span>
MsgBox [HM2]
Range(<span style="color:#800000"; >"$A$1"</span>).ClearContents
<span style="color:#000080"; >End</span> <span style="color:#000080"; >If</span>
<span style="color:#000080"; >End</span> <span style="color:#000080"; >If</span>
Application.EnableEvents = <span style="color:#000080"; >True</span>
<span style="color:#000080"; >End</span> <span style="color:#000080"; >Sub</span>
</span></nobr></div>