In previous works, we have postulated the existence of an immunological equilibrium between persistence of the antigen presence and the B-cell maturation-differentiation; from this hypothesis, it follows that the antigen presence induces the IgM to IgG switch and the final exhaustion of the response which persists if the antigen stimulation persists. On the basis of those results, we have considered of interest to study the direct and indirect polyclonal activation of the background of murine plaque-forming cells (PFC), against sheep erythrocytes. We have investigated this activation by different doses of mitogens, which activate B cells, thus producing various cellular proliferations. The goal was to find out whether polyclonal activation increased the background (as expected) but did not change the IgM/IgG PFC ratio, probably because of a lack of antigenic stimulation. As mitogens, we have used: E. coli lipopolysaccharide (LPS), a known polyclonal activator of B cells which causes a proportional increase in the number of background plaque-forming cells to a given antigen, and pokeweed mitogen (PWM) which, although less frequently used, has been shown to stimulate both T and B lymphocytes. Female Swiss albino mice have been used; 10 mice per group for both control and test groups. The mitogens were administered according to the following doses and protocol: LPS-1 dose of 10, 50 or 100 micrograms; 3 doses of 10, 50, or 100 micrograms (1 dose per week). PWM-1 dose of 50, 100 or 200 micrograms; 3 doses of 50 or 100 micrograms (1 dose per week).(ABSTRACT TRUNCATED AT 250 WORDS)