Antibodies to prostaglandin were obtained by immunization of rabbits with PGA(1), PGA(2), and PGE(1), protein conjugates of prostaglandins. The antibodies demonstrated specificity toward both the cyclopentane ring and the aliphatic side chains. With the use of these antibodies a highly sensitive radio-immunoassay capable of measuring less than picomolar amounts of PGA1, PGA2, and PGE1 has been developed.