Production of 17.2.25 p transgenic and endogenous immunoglobulin in X-linked immune deficient mice* In MS4 mice transgenic for a completely rearranged pa heavy chain there is a decrease in total B cells and the rearrangement of endogenous immunoglobulin genes is partially inhibited. Surprisingly, however, endogenous immunoglobulin gene rearrangement and significant heavy chain polypeptide production does occur. We tested the hypothesis that only CD5+ B cells produce endogenous immunoglobulin by taking advantage of the fact that X-linked immune deficient (xid) mice normally are deficient in CD5+ B cells. We found that the frequency of CD5+ splenic B cells was similar in XXIdY transgenic and non-transgenic F1 males, and in XXldX transgenic and non-transgenic F1 females. In both XXIdX and XXIdY transgenic F1 mice some, but not all, splenic B cells are CDllb+. There was a striking deficit of splenic B cells expressing endogenous immunoglobulin in XXIdY transgenic mice, although this was not true for peritoneal cells. Thus, the introduction of the 17.2.25 p transgene does not prevent the development of CD5-B cells nor does it limit endogenous immunoglobulin gene arrangement and expression solely to CD5+ B cells. However, in mice capable of expressing B cell surface CDS or C D l l this transgene can lead to expansion of the fraction of B cells positive for these molecules. We conclude that while the introduction of the 17.2.25 p transgene alters the frequencies of B cell populations, maturation is not limited to one subpopulation.