AbstractAPSTRACTA lyophilized extract from F. coli (0M-89) was studied for its immunomodulating properties and tolerance in humans. Its oral administration to healthy volunteers produced a selective increase in the active T-cell population without changes in other lymphocyte populations. A significant increase in the proliferative response to concanavalin A and phytohemaaqlutin was recorded, hut not to pokeweed mitogen. No significant changes were observed in the serum levels of IgG, IgA and IgM. The clinical and biological tolerance of OM-89 was excellent, without any adverse side-effects or production of circulating immune complexes or of autoantibodies, while the in vitro investigation showed that it is not a mitogen. Thus in healthy subjects 0M-89 seems to act mainly on the cell-mediated immune responses.