SCID mice as they age may develop a limited number of functional B and T cell clones. VH and VL gene sequences of hybridoma antibodies derived from the spleens of five adult SCID Ig+ mice revealed nine clonally unrelated, i.e. derived from different precursor lymphocytes, groups of Ig. Of these, four used the VH7183 family, one Q52, three J558 and one VH3609. VH7183 was seen in excess over J558 despite a 5-fold larger number of J558 family members in the mouse genome. Preferential use of D proximal VHgenes along with reduced N region addition in the heavy chain VDJ joins of these antibodies suggest derivation from a population of lymphocytes with characteristics of the perinatal repertoire. All of the hybridomas produced κ light chains of several different VL family groups. Sequences of 23 VH and 22 VL genes of hybridomas derived from one mouse (no. 45) showed that they were all clonally related, differing only in somatic mutations. Autoreactivity of these antibodies to cellular antigens [including (U1)RNP-specific A protein and SmB] was analyzed, and shown to correlate with the geneology and specific mutations within members of the clone. The geneological tree derived from sequence data permitted a study of the microevolution of a clone of B cells and showed that as it developed there was a tendency toward increased strength of self-reactive binding as well as alterations in specificity toward these autoantigens. Analysis of replacement: silent mutation ratios within members of this clone reflected limited affinity maturation in mouse 45 and none for another mouse, no. 58. These data are consistent with the progressive differentiation of B cells in an immunodeficient state in the absence of clonal competition with B cells of similar antigen specificities as they populate the spleen. © 1995 Oxford University Press.