Adding Conditional Column values ?

Hey
I am new to excel, I need to add column A values. Some thing like this will give me complete total of column A =SUM(A1:An). How will I do a conditional test. I need to add only those column A values whose Column B values is AB
A B
9 DT
1 DT
1 DT
1 AB
1 DT
1 DT
1 AB
7 DT
1 DT
1 AB
1 DT
. .
n n


Thanks
Asgar