Infection with filarial nematodes is commonly associated with a failure of T cells to proliferate in response to parasite antigen. We have investigated the possibility that antigen-presenting cells recruited during filarial infection are responsible for impairment of T cell function. We have found that the human filarial parasite <i>Brugia malayi,</i> when implanted into the peritoneal cavity of mice, recruits a population of adherent cells that actively block the proliferation of T cells. Phenotypic analysis of the recruited cells reveals large numbers of both macrophages and eosinophils and cell sorting experiments demonstrate that these antiproliferative cells are Mac-1-positive. Studies in gene-deficient mice have demonstrated that proliferative suppression is dependent on the in vivo production of IL-4 but not IL-5 or IL-10, while suppression in vitro is not mediated by any known antiproliferative factor. Our results suggest that helminth infection can lead to the development and/or recruitment of an IL-4-dependent macrophage population that mediates suppression via a novel mechanism.