Summary: The relationship between somatotropic hormone (STH) and the development of lymphoid organs was investigated. Repeated injections of rabbit anti-Raben-type bovine somatotropic hormone (ASTH) globulins into 2- and 3-week-old Charles River mice produced body growth inhibition, thymic atrophy, involution of the lymphoid tissue in the spleen and a wasting syndrome. Similar treatment with normal rabbit serum (NRS) globulins produced a normal splenic cellular reaction and no wasting syndrome was observed. The same treatment in adult male mice produced similar effects on the lymphoid organs but no wasting syndrome. The simultaneous administration of STH to mice treated with ASTH globulins completely reversed the effects of ASTH globulins. Histologic examination revealed that the ASTH treatment resulted in disappearance of small lymphocytes in the cortex of the thymus, in the perifollicular mantles of the germinal centers and in thymus-dependent areas in the spleen. These results are discussed in relation to the hypothesis that wasting disease in neonatally thymectomized mice derives from disruption of a critical hypophysis-thymus axis in the perinatal age. A role for STH and other hormones for differentiation of antibody-producing cells is proposed.