AbstractWe investigated the effects of an Escherichia coli-derived product (OM-89) in mice. The oral administration of OM-89 led to a significant (p<0.05. Student's t test) increase in the levels of IgA in intestinal secretions, which was at maximum 25 days after the end of the treatment, when a two-fold increase in IgA levels was observed. The i.p. inoculation of OM-89 induced the stimulation of anti-SRBC plaque-forming cells (PFC) in the spleen. The effect of OM-89 was dose-dependent and produced up to a 9-fold increase in PFC in the treated mice when compared to untreated controls. The oral administration of OM-89 proved to be effective in the enhancement of resistance to challenge i.p. inoculation with E. coli. 32% of OM-89-treated mice showed resistance to this experimental infection at minimal LD100. The combined effects of low environmental temperature and cyclophosphamide (CY) immunosuppression enabled us to enhance differences in survival rates in experiments on the modulation of resistance towards Pseudomonas aeruginosa infection. The oral treatment with the immunoraodulator induced a significant (p<0.05, Student's t test) level of protection in CY-immunosuppressed mice to the intranasal infection with P. aeruginosa, when mice were kept at low environmental temperature right after the bacterial challenge. The protective effect of OM-89 treatment was dependent on both the environmental temperature and the timing of the experiment.