VS 2010 event in form1 to trigger something in form2

I'm trying to do something in form2 when the SerialPort1 in form1 gives the DataReceived event.
To simplify the problem I'm trying to have a button in form1 to open a msgbox with for instance "action in form2" in form2.

Still I'm struggeling to have the button1_clicked event in form1 activating the msgbox of form2...
How do I create this global event?