The only strains of mice which are able to synthesize lambda 1-bearing antibodies in response to alpha (1-3) Dextran are those expressing the Igha allotypic haplotype or those having an Igh V region identical to Igha mice. The experiments reported here were designed to investigate whether the nonresponsiveness of mice which do not express the Igha haplotype is a consequence of an absence of a polyclonal B cell receptor for the alpha (1-3) Dextran TI-antigen. B cells of several mouse strains were stimulated with polyclonal B cell activators (PBA) known to either stimulate non-overlapping B cell subsets or to stimulate B cells at different stages of maturation, i.e., lipopolysaccharide, Nocardia delipidated cell mitogens and alloreactive T helper cells. Whereas all three PBA induced B cells from Igha mice to secrete lambda 1-bearing anti-alpha (1-3) antibodies, the PBA were incapable of inducing B cells from non-Igha mice to mount an anti-alpha (1-3) Dextran response. The data suggest that non-Igha mice lack a functional VH Dex gene for the lambda 1-bearing anti-alpha (1-3) Dextran response.