Tim Zych wrote:

If the macro workbook (A) is referencing the VBAProject of the other
workbook (B), B will open when A is opened.

This can be seen in the VBE. In the project window of A, there would be a
References treeview. Alternatively, it can be seen in the VBE's Tools -
References with a check next to the VBA project of B. It may be named
"VBAProject" or something else.

Could have been it, but there's no reference to a particular VBA project in Tools References. Also, both Xlstart folders are empty. And if I close B, and run or step through the Auto_Open() procedure again, B does not open again! What else can I do to track it down?

Gustaf