A B S T R A C T F, hybrid offspring of New ZealandBlack mothers and New Zealand White fathers [(NZB X NZW)F1] female mice develop antibodies to singlestranded (ss) and native DNA, immune complex glomerulonephritis, massive proteinuria, and premature death with renal failure. By a series of matings, congenic (NZB X NZW)Fj * xid/xid mice were prepared. These mice were different from (NZB X NZW)Fj mice in having the X chromosome-linked immune deficiency gene, xid, in homozygous form. Such congenic (NZB X NZW)F, xid/xid females failed to develop antibodies to single-stranded or native DNA. They also failed to develop fatal renal disease as measured by proteinuria, glomerular histology, glomerular immunofluorescence, and survival.To control for unknown genetic factors, studies were performed with littermates that were derived by mating NZB * xid/+ females with NZW * xid/Y males such that the resulting offspring were either (NZB X NZW)F1. xid/xid (and therefore "defective") or (NZB X NZW)F1 . xid/+ [phenotypically like (NZB X NZW)Fl]. In these and in additional studies, mice were housed in the same cages and identified by ear tagging so as to avoid possible environmental variations from cage to cage. In these studies, xid/xid mice failed to develop the characteristic signs of autoimmunity, whereas the controls did. Similar results were also obtained with (NZW X NZB)F1 xid/xid mice compared with (NZW X NZB)Fj xid/+ mice.The effect of xid/xid upon (NZB X NZW)F1 mice was further investigated by assessing responses to immunization and polyclonal B cell activation in vivo.