Passing arguments to a macro in another file

Hi,

I use the following macro "modifystri" in a workbook "Modify" to pass a variable to a macro ("macro1") located in a sheet ("Sheet1") in another workbook ("Test.xls")

sub modifystri ()
dim stri as string
Call Application.run "Test.xls!Sheet1.macro1", stri
end sub

The definition of macro1 located in workbook "Test.xls" is:

sub macro1 (stri as string)
      stri = "ABC"
end sub

The value of stri after calling "macro1" is still empty. I want to use "macro1" to modify the contents of stri. Please help.