Summary The incidence of intercurrent bacterial infections and wasting was considerably higher among neonatally thymectomized rats than in splenectomized or sham-operated controls. Administration of oxytetracycline or implantation of thymus auto-grafts immediately after thymectomy markedly reduced the incidence of bacterial infection and wasting in thymectomized rats. Oxytetracycline-treated thymectomized and control animals showed a considerably more effective antibody response to horse serum than untreated controls. Bacterial infection, usually in the form of abscesses in the region of the ablated thymus or bronchopulmonary inflammation, was noted in all wasted animals. A predominantly plasmacytic inflammatory reaction and elevation of serum gamma globulin level were observed in most wasted neonatally thymectomized rats in spite of their lymphopenia and splenic atrophy. The findings suggest that bacterial infections, presumably acquired early in life, contribute to the subsequent development of immunologic deficiency and wasting in neonatally thymectomized rats. The able assistance of Julia Williams and Dolly Houston is gratefully acknowledged. The author is also indebted to Lucille Seguin and William Robinson for performing bacteriological tests.