The compensatory effect of a bacterial lysate, Broncho-Vaxom (BV) on the immunosuppressive action of cyclophosphamide (CY) was investigated. In CY immunosuppressed mice, BV treated animals recovered to normal levels of IgM and IgG in serum as well of IgA and IgG in gut secretions significantly earlier than controls. Furthermore, normal cell proliferation in thymus, as estimated by measuring the relative size of this organ was achieved earlier in BV treated mice than in control mice. Oral treatment with BV restores the number of IgM anti SRBC producing cells in spleen, in CY immunosuppressed mice. Since immunosuppression induced by CY increases the susceptibility to various infections, we tested in immunosuppressed animals the protective effect of BV towards IP challenge infections with Streptococcus pneumoniae, Staphylococcus aureus, Klebsiella pneumoniae var ozaenae, Pseudomonas aeruginosa and Candida albicans. BV led to an enhanced resistance towards both pneumococci and staphylococci challenge infections but not to the other challenge microorganisms.