A single gene mutation results in near absence of B and T lymphocytes and their immediate progenitors in mice with severe combined immunodeficiency syndrome (SCID). However, long term culture conditions allowed rapid outgrowth of lymphocytes from SCID bone marrow suspensions, and this permitted their detailed analysis. The cells were judged to be committed to the B lymphocyte lineage on the basis of expression of the BP-1 antigen, as well as by the density and pattern of expression of other markers. Cultured SCID lymphocytes were indistinguishable from control BALB/c cells in terms of morphology, typing for 13 cell surface markers, and changes in cell surface antigen expression with time in culture. In contrast to cultures of normal cells, which always included IgM synthesizing cells, SCID lymphocytes rarely expressed μ heavy chains. Southern blot analysis demonstrated that at least the first Ig gene rerarrangement step had occurred in most of the cultured cells. The patterns of J(H) gene rearrangements suggested that relatively limited population diversity existed in individual cultures of SCID and normal BALB/c marrow. In addition, there was eivdence that abnormal Ig heavy chain gene rearrangements had taken place in lymphocytes from approximately 25% of the SCID cultures. These cells were distinguished by the absence of detectable J(H) gene segments. κ light chain genes appeared to be unrearranged in SCID cultured lymphocytes. We conclude that the lymphopoietic microenvironments of SCID mice are probably normal, and the animals have infrequent progenitors of B cells. Aberrant or nonproductive IgH gene rearrangements may account for the absence of pre-B and B cells in SCID mice. This study demonstrates the usefulness of long term culture methodology for isolating rare subsets of non-transformed lymphoid cells from normal and genetically defective hemopoietic tissues.