The present invention provides an immunoassay for phthalic acid esters, which comprises measuring the phthalic acid esters contained in a sample using an antibody produced with a conjugate of a carrier protein and a phthalic acid ester derivative represented by the formula
wherein R1 and R2 may be the same or different and are hydrogen, alkyl, cycloalkyl or phenylalkyl, with the proviso that both of R1 and R2 are not hydrogen; m is an integer from 1 to 5; and Y is amino or carboxyl.