Carboxymethylglucan (CMG) with ultrasonically lowered molecular weight (0.89 x 10(5)) was administered either intraperitoneally, intravenously or orally prior to cyclophosphamide (CP) injection and its effect on the frequency of micronuclei in mouse bone marrow was evaluated. Both parenteral (intraperitoneal and intravenous) and oral administration of CMG decreased the clastogenic effect of CP. The protective effect induced by intravenous and intraperitoneal administration was concentration-dependent, with a higher decrease achieved by 200 mg/kg than by 100 mg/kg body weight. With the lower dose of CMG a 2-h interval was necessary between intravenous CMG administration and CP injection. Oral pretreatment of mice with CMG decreased statistically significantly the frequency of micronuclei in polychromatic erythrocytes of the bone marrow. The fact that ultrasonically depolymerized CMG was effective also on oral administration is indicative of the passage of smaller CMG molecules through the wall of the gastrointestinal tract.