SUMMARYThe ♂ antigen in the rat is described. The median survival times (MSTs) of intra-strain ♂ → ♀ skin grafts vary between 50 and 280 days. No association is apparent between ♀ reactivity to the ♂ antigen and theAg-B (Rt-H1)genotype. The ♂ antigen is identical in 6 of 7 strains so far investigated. The MSTs of successive ♂ → ♀ skin grafts become progressively shorter for the first four grafts, but thereafter tend to fluctuate in duration; considerable individual fluctuation is evident over a series of grafts. At no stage does rejection proceed with the maximum intensity of which the host is capable. The MSTs of parental strain ♂ skin grafts on F1hybrid ♀ hosts are intermediate between the MSTs recorded for ♂ → ♀ skin grafts in the two parental strains. Skin from ♂ donors shows a progressive increase in immunogenicity between birth and old age. This is probably an attribute of antigenic weakness rather than a specific characteristic of the ♂ antigen. Neonatal ♂ skin grafts effect greatly prolonged survival of simultaneously grafted aged ♂ skin from the same strain, the effect being immunologically specific.