Did you go to Tufts, by any chance?
You should call your registrar's office and ask how many credit hours each class is worth (remember to tell them that the class had a lab component). Whatever value they tell you is the number that you enter into the "credit hours" section of the AMCAS grade report (also enter the grade as it appears on your transcript).
I think that, for the most part, science classes count for 4 credit hours.
Good luck!
mma