Mixed bacterial toxin (MBT) is an experimental substance for treatment of certain forms of cancer. To ascertain if the cancer regression affects reported for MBT might be mediated via immunoendocrine interactions, MBT was tested in vivo in male Wistar rats or in vitro on Thymic (Tc) or Splenic (S) lymphocytes prepared from these same animals. In order to evaluate gonadal steroid effects on this process, certain groups of these rats were also castrated before treatment. Tc or S lymphocytes were prepared from these groups and cell responsiveness was monitored by in vitro blastogenic assays. Certain assay wells also contained Concanavalin A as a mitogen. The addition of MBT in vitro significantly stimulated Tc-cell responses (p less than 0.05) for all experimental groups except for certain groups of castrate MBT injected rats. Furthermore, S cells did not respond in the presence of MBT added in vitro except after castration and pretreatment with MBT where in vitro addition of MBT to cultures significantly stimulated blastogenic responses (p less than 0.02). The results of these assays are consistent with the hypothesis that the anticancer effects attributed to MBT result from the ability of MBT to act as a nonspecific mitogen-like stimulator of immature Tc-cells and mature S-cells. These results also suggest that MBT may modulate a serum factor under the control of gonadal steroids and possibly of thymic origin which can effect T-cell responsiveness.