Prostatic acid phosphatase can be detected in individual human prostatic fluids by means of gel diffusion antigen-antibody precipitation with specific staining procedures. The fluids can be diluted to various degrees before the reaction fails. This titre, or limiting dilution, ranged from 1 : 64 to 1 : 16,000. It paralleled the results obtained by the King-Armstrong assay of enzyme activity. Gel diffusion analysis revealed the presence of two antigens with acid phosphatase activity.