Studies were made to investigate the quantitative and functional changes which occur in peritoneal macrophage populations obtained from mice infected orally with Trichinella spiralis larvae. C57BL/6 mice infected with T. spiralis larvae became parasitized with adult worms which were rejected from the intestine from 14 to 20 days after infection. Infected mice developed a striking increase in peritoneal exudate cells, composed largely of macrophages, which was maximal at from 16 to 18 days after infection. T. spiralis larvae and eosinophils were not seen in the peritoneal exudates. Macrophages from mice infected more than 11 days earlier inhibited DNA synthesis of syngeneic and allogeneic tumour cells, a property atributed to activated macrophages. In addition, macrophages from T. spiralis-infected mice had the functional ability to kill EL-4 tumour cells as measured by 51Cr release. Unlike activated macrophages, however, macrophages from infected mice did not develop the ability to inhibit multiplication of the intracellular pathogen Toxoplasma gondii. These studies demonstrate that T. spiralis infection in mice induces changes in macrophage function that differ from changes associated with infections by intracellular pathogens.