The authors evaluated the in vitro effects of Bacillus subtilis on the following parameters of the immune response: mitogenic T cell proliferation (by PHA and OKT3) and mitogenic-induced lymphokine production (IL-2 and IFN-gamma). The spores of Bacillus subtilis did not influence the immune response, while its vegetative forms enhanced mitogenic-induced T cell proliferation. Both spores and vegetative forms did not modify lymphokine production.