With Rs
    .CursorType = adOpenKeyset
    .LockType = adLockPessimistic
    Select Case ComboBox1.ListIndex
        If Filter Is Nothing Then
            MsgBox ("Ungültige Filter-Werte!")
        Else
            Case 0
                .Open "SELECT * FROM prod_definition WHERE PD_NUM LIKE '" & _
                  Filter & "'", Cn
            Case 1
                .Open "SELECT * FROM prod_definition WHERE PD_BEZ LIKE '" & _
                  Filter & "'", Cn
            Case 2
                .Open "SELECT * FROM prod_definition WHERE M_Bestelltext LIKE" & _
                  "'" & Filter & "'", Cn
        End If
        If Rs.Count = 0 Then
            MsgBox ("Nix gefunden!")
        End If
    End Select
End With

VG

Dirk