T and B lymphocyte activity in adoptive secondary antibody responses was assessed by cell titration in mice of the X-linked immune defective CBA/N strain and mice of their normal partner strain CBA/CaJ. No quantitative differences could be detected in either T or B cell activity in these experiments, in which 2,4-dinitrophenyl was used as the hapten. Both of the helper T cells detected in such assays were present in both strains. During these experiments, it was found that a subline of CBA/N maintained at Yale may have diverged from the CBA/N NIH stock. This was detected by an ability of irradiated mice of the Yale subline to increase the antibody response of B cells from the NIH subline, provided specific helper T cells were also present. This allogeneic effect apparently replaced the activity of one helper T cell, but did not replace the requirement for a carrier-specific helper T cell.