Following transplant, circulating immunoglobulin levels fell moderately and remained depressed less than 2 months for IgG, and for variable and longer periods of time for IgM and IgA. Repeated quantitative determinations of antibodies against multiple antigens did not show any decrease in the pretransplant levels. Indeed some patients developed herpes and cytomegalovirus infections to which they responded by a sharp increase in antibody titers. In 2 cases, a primary immunization was demonstrated (against CMV and BK virus) with increasing levels of IgM and IgG antibodies. Lymphocyte counts in peripheral blood returned to 500 mm# between day 10 and 29 (median day 18) and to pretransplant values within 6 weeks. Non specific stimulation of lymphocytes by mitogens in the immediate post-transplant period showed a decreased response to PHA and Con A, whereas the responses to pokeweek mitogens and alloantigens were only slightly diminished. The degree of the responses was related to the dose of cryopreserved marrow infused. We conclude that:--although the minimum dose for autologous bone marrow transplantation in man is around 0,5 10(8) nucleated bone marrow cells/Kg, much higher doses should be used to ensure faster and better restoration of immune reactivity.--The similarity of the immunological dysfunction following autologous and allogeneous bone marrow transplantation suggest that, in the immediate post-transplant period, the role of GVHD in cellular immunity depression may be minimal.