The immunomodulating effect of an extract of the cell wall of Mycobacterium sp. (WSF, Vetrepharm Inc., London, Canada) in foot-and-mouth disease virus inactivated vaccines was tested in a murine model. The duration of immunity, protection, stimulation of immunocompetent cells acting on the long-lasting secondary response and possible tissue damage were examined. The incorporation of 10 micrograms WSF into aqueous and oil vaccines induced a high and long-lasting specific antibody response. The neutralizing titres of these antibodies were significantly higher than those observed in animals immunized with vaccines lacking WSF and conferred protection for at least 7 months. The data collected in repopulation assays indicate that WSF participates in the activation of immune cell populations involved in long-lasting memory. This results in an efficient B-cell secondary response even in the absence of T cells, lasting at least 5 months. No adverse reactions were detected. The enhancement effect of WSF on the immune response to foot and mouth disease vaccines observed in the murine model indicates the possibility of its inclusion in aqueous vaccines to be tested in cattle.