A 14-day-old Chinese male baby was admitted with extensive skin lesions. A wound culture grew Staphylococcus aureus, Acinetobacter anitratus, Enterobacter cloacae, and Candida albicans and a blood culture grew group A beta-Streptococcus hemolyticus. The patient's lymphocyte counts were low and his lymphocytes were unable to produce IgG and IgA in vitro. The immunoglobulin-bearing cell studies also failed to demonstrate IgG and IgA bearing cells. Active Tac+ T cells, total T cells, and T cell subsets were at very low levels. Lymphoproliferative response to mitogens was also poor. Migration inhibitory factor production to Candida antigen was also decreased. The initial lymph node biopsy demonstrated no follicular formation and extensive depletion of lymphocytes in both thymic-dependent and thymic-independent areas. After Thymostimulin (a specific bovine thymic extract, TP-1) treatment, the second lymph node biopsy demonstrated germinal centers containing IgA-bearing cells and IgM-bearing cells and, subsequently, cortical and medullary differentiation. Serum IgG, IgA, and IgM became detectable at low levels and IgG-, IgA-, and IgM-bearing lymphocytes appeared in the peripheral blood. This also correlated with in vitro immunoglobulin synthesis. Active Tac+ T cells, total T cells, T cell subsets and lymphoproliferative response to mitogens increased gradually after thymostimulin therapy. This investigation demonstrated the therapeutic effectiveness of Thymostimulin in combined immunodeficiency both histologically and immunologically and the successful reconstitution of B cell function that did not require continued therapy.