Anti-H-Y antiserum is generally obtained from female inbred mice or rats that have been hyperimmunized with syngeneic male cells. The specificity of such antiserum is defined by its reactivity for male but not female cells. A number of conventional serological assays have been used to measure that reactivity. However, H-Y is a weak antigen, evidently represented sparingly on the surfaces of cells other than sperm, epidermal cells and brain cells ; thus the serological assays for H-Y are technically difficult. Yet H-Y serology has enabled significant progress toward the understanding of primary sex differentiation. A recent advance in H-Y serology is the establishment of monoclonal anti-H-Y antisera which promise to facilitate analysis and clarification of the H-Y system.