Metallothionein (MT) is a thiol‐rich protein that is rapidly induced by exposure to heavy metal cations. We have previously demonstrated that exogenous MT stimulates murine splenocytes to proliferate, but inhibits humoral responses to antigen. These observations suggest that metallothionein released from cells has a complex role in heavy metal‐mediated immune dysfunction. Here we examine one possible mechanism by which MT mediates suppression of humoral immunity. Exposure of macrophages to 20 μM MT did not affect their ability to engulf opsonized sheep erythrocytes, but in the presence of 20 μM MT, peritoneal macrophages were stimulated to produce increased levels of oxygen radicals. These results correlated with observations that while macrophage phagocytosis of opsonized Candida albicans was unaltered by the presence of exogenous MT, killing of the engulfed yeast cells was dramatically enhanced by 20 μM MT. Amounts of free cadmium and zinc equimolar to that added as Zn,Cd‐MT had no effect on candidacidal activity. MT was also found to significantly decrease lymphocyte proliferation mediated by macrophage activity. Biotinylated MT (MT‐b) bound specifically to the plasma membranes of these macrophages, suggesting that membrane‐associated molecules of the macrophage may transduce a signal mediated by MT binding. These results demonstrate that macrophages are a sensitive target for MT‐mediated immunomodulation and that some of the consequences of the MT interaction with macrophages may be alterations in the capacity to produce an effective immune response and increased extracellular exposure to damaging free radicals.