Fecal pellets of mice infected with Trichinella spiralis, Hymenolepis nana, and Schistosoma mansoni have been found to contain high levels of phospholipase B activity. The rise, time course and decline of the enzymatic content of the pellets correlate with the known patterns of intestinal injury and reaction due to the parasites or their eggs. Treatment with drugs (thiabendazole, niclosamide, niridazole) which are effective in suppressing the infection also prevents the rise, or causes an early decline, in the titers of phospholipase B appearing in the excreta. These findings complement the previous reports of a close correlation between accumulation of this enzyme in the intestine and infection of mice with T. spiralis and H. nana. Determination of fecal phospholipase B activity constitutes a relatively simple, quantitative, and blood-free method of following the course of infection and its response to treatment, which might be of particular advantage in long term studies and preliminary therapeutic screening.