A counter immunoelectrophoresis procedure for the measurement of acid phosphatase is described. Acid phosphatase was purified from human prostate. A monospecific antiserum to the purified enzyme was produced in rabbits. With the specific antiserum and a sensitive chemical staining technique, a counter immunoelectrophoresis assay was developed for detection of serum prostate acid phosphatase. The isoenzymes of serum prostate acid phosphatase were also studied by an isoelectric focusing technique.