I agree with the other black_rose and Liz... Let the other mummy know and then let her decide. I'm usually pretty chilled about it all, if the other kids snot is clear and the mummy thinks the kid is doing ok then I'm all up for a playdate. However, I have a mummy friend who will cancel things if she knows that my DS has been sick or has a runny nose or whatever because she is super paranoid about her DD getting sick. I accept that, no biggie