The phagocytic ability of polymorphonuclear leukocytes, the metabolic activity of peritoneal macrophages, the proliferative response of T- and B-cells, and the production of specific anti-Toxocara antibodies were studied in paratenic hosts with experimental larval toxocarosis after treatment with the immunomodulator muramyldipeptide for 119 days. Peroral infection of mice with 2,500 Toxocara canis eggs partially reduced the numbers of cells and inhibited the activity of all cellular immune-response parameters studied. During most of the experiment the greatest suppression was recorded for the peritoneal-macrophage metabolic activity. Parenteral administration of muramyldipeptide to mice at two doses prior to and after infection restored and significantly stimulated the parasite-inhibited phagocytic ability of blood polymorphonuclear leukocytes, metabolic activity of macrophages, and T-lymphocyte proliferative response. It also elevated the production of specific circulating anti-Toxocara antibodies. The immunomodulator significantly reduced the count of migrating T. canis larvae in the host organism (30.6%) and decreased by half the percentage of larvae in the brain.