B-cell antibody secretion by lymphocytes from 50 bone marrow transplant recipients and 42 healthy controls was studied in vitro using an indirect hemolysis-in-gel assay to determine Ig-class-specific plaque- forming cells (PFC). The numbers of PFC were determined in medium alone and after stimulation of lymphocytes with killed Staphylococcus aureus bacteria. The PFC responses for IgG, IgA, and IgM after stimulation with S. aureus were significantly lower in patients studied during the first 101 days after grafting compared to normals. Statistically significant increased IgG-PFC were found in patients who had acute graft-vesus-host disease (GVHD), grafts from HLA-nonidentical donors, or infections. Healthy patients studied more than 1 yr following transplantation had normal B-cell responses, but patients with chronic GVHD had deficient IgM production. The data suggest that antibody secretion by B cells varies in different marrow transplant patient subgroups and present a basis for further investigation of the interaction between lymphocyte subpopulations leading to antibody production.