Mice with the autosomal recessive severe combined immune deficiency (scid) mutation lack mature lymphocytes because of defective joining of T cell receptor and immunoglobulin (Ig) gene segments. Penetrance of this mutation is incomplete since 10-25% of SCID mice produce some T or B lymphocytes. This "leaky" phenotype could be due to a reversion of the mutation in some mice or to a constant, low frequency of functional lymphocytes generated in all SCID mice with variable survival of such cells. We report here that all SCID mice can be stimulated to produce functional B cells by the transfer of normal neonatal, but not adult, T cells. T cell-induced rescue of C.B-17scid B cells results in high levels of Ig expressing the Ighb allotype of the SCID recipient. These results show that all SCID mice generate some functional B cells, the majority of which do not survive in the absence of a subset of T cells present in high frequency in the neonate.