N-acetylmuramyl-l-alanyl-d-isoglutamine (muramyl dipeptide) stimulates in vitro primary immune responses to SRBC in T cell-depleted (nude) spleen cultures. The stimulation of immune responses by muramyl peptide was antigen dependent. A microculture system was used to compare the T cell-replacing activities of several structural analogues of muramyl dipeptide and to compare the activity of muramyl dipeptide to helper T cells. In a limiting dilution analysis with excess helper T cells or muramyl dipeptide, the frequency of B cell precursors that respond to SRBC was similar, ranging from 1.5 to 5 × 10-5. Decreasing the cell density in microcultures did not affect the efficiency of B cell precursor responses in the presence of muramyl dipeptide. Muramyl dipeptide was examined for mitogenic activity in spleen cell cultures. In serum-free medium, muramyl dipeptide stimulates slight (3-fold) increases in DNA synthetic activity. In medium supplemented with 5 to 20% fetal calf serum, muramyl dipeptide showed no significant mitogenic activity. There are a number of possible explanations for the T cell-replacing activity of muramyl dipeptide. The most likely is that muramyl dipeptide interacts directly with B cells to mimic the helper T cell signal in the inductive stimulus.