SUMMARY Separation of the subunits of human pituitary luteinizing hormone (LH) has been accomplished by dissociation of the hormone with 8 m-urea followed by chromatography on CM-cellulose at pH 5·0 or by counter-current distribution. Each procedure resulted in isolation of one subunit, the other subunit being contaminated with reassociated LH. The isolated subunits (α and β) are each of low biological activity, but when recombined in a 1:1 ratio at pH 7, reassociation occurs with restoration of full activity. The amino acid compositions of the two subunits are significantly different from each other, but the composition of the α subunit is very similar to published data on the corresponding subunit of human chorionic gonadotrophin (HCG) and human thyroid-stimulating hormone. Significant differences were observed when comparing the compositions of the β subunits of the hormones. By immunoradiometric assay using antibodies to HCG, the α subunit was as active as native LH on a weight basis, but the β subunit was significantly lower in immunological activity.