Mice of the HRS strain, which carry the mutant gene hr, were examined for abnormalities in representation of the three T-cell sets Ly1, Ly23, and Ly123 in the spleen. The salient feature of hr/hr mice, which are immunologically deficient, in comparison with +/hr segregants, was a gross disproportion in numbers of cells belonging to the Ly1 and Ly123 sets, at the age of 3--3.5 mo. At this age, Ly123 cells of hr/hr spleen outnumbered Ly1 cells by 2:1, whereas in +/hr spleens Ly123 cells were outnumbered by approximately 1:2. Cells from pooled lymph nodes of hr/hr mice did not show a correspondingly gross disporprotion of Ly1 and Ly123 cells. Total counts of splenic T cells, and of B cells, were not significantly different in hr/hr and +/hr mice.