Neonatally thymectomized hamsters implanted with diffusion envelopes containing thymus showed partial prevention of the effects of thymectomy. A normal humoral antibody response was maintained in these animals, although a significant lymphocytopenia remained. This suggests that the thymus participates in immunogenesis by a humoral mechanism and possibly by a cellular mechanism as well, which was blocked by the diffusion envelope. Thymectomized hamsters implanted with diffusion envelopes containing adult spleen, bone marrow or kidney were still impaired in their ability to produce humoral antibody, although these tissues appeared to enhance antibody production. The thymus humoral factor appeared to be specific to the thymus. Reimplanted diffusion envelopes, containing large thymic cells resembling epithelial-reticular cells, were used to show that these cells produced the thymus humoral factor.