Relatively specific antisera were raised to the subunits of human luteinizing hormone. AH 5 rabbits immunized with 50 μg hLH-β produced antisera which discriminated between hLH and hCG as evidenced by differences in slopes of their respective inhibition lines.Human, ovine, bovine and rat LH subunits, prepared by counter-current distribution, were studied in the anti-hLH-α and anti-hLH-β radioimmunoassay systems along with hFSH, hCG and their subunits. Whereas ovine, bovine and rat LH-α failed to yield any inhibition in the antihLH-α radioimmunoassay system, hFSH-α and hCG-α showed complete cross-reactivity with hLH-α.On the other hand, ovine, bovine and rat LH-β showed complete cross reactivity with hLH-β in the anti-hLH-β radioimmunoassay. However, hCG and its β subunit showed incomplete cross-reactivity.Our results suggest that 1) inter-species cross reactivity resides in the P subunits of those hormones, 2) within species cross-reactivity probably resides in the α subunit, and 3) specific antisera to the glycoprotein hormones are more likely to result with immunization with the hormone-specific β subunit. (Endocrinology 91: 1337, 1972)