The effect of glucan, a biological response modifier of yeast origin, on different immune functions was studied after the intramuscular application in goats. The simultaneous administration of glucan with human serum albumin or tetanus toxoid significantly stimulated the antibody production to both antigens differing in their thymus dependency. Similarly, the phagocytizing activity of the blood polymorphonuclear leukocytes measured by the reductase colorimetric assay significantly increased one week after the glucan treatment. However, suppression of T-lymphocyte function in experimental animals was determined by the lymphocyte transformation test particularly in response to phytohemagglutinin and concanavalin A. The results of the study indicate that glucan can modulate some elements of the ruminant immune response.