unlike laboratory animals , people are rarely exposed to a single hazardous chemical . 
 however , most of the information documenting adverse human health effects from environmental and occupational contaminants has come from studies focused on exposure to single chemicals , and there is little information available on how two or more contaminants affect humans . 
 most information on the effects of mixtures comes from animal systems and limited investigations of isolated human cells in culture , even though the study of mixtures in such systems has also been neglected . 
 two or more compounds may show additive , antagonistic , or synergistic interactions or may act on totally different systems and thus not interact . 
 furthermore , even a single chemical may have multiple effects and affect more than one organ system . 
 effects may vary with age , and metabolites may have totally different actions from the parent compound . 
 this paper will review the variety of health effects in humans that may result from environmental contaminants and discuss how such contaminants may interact with each other . 
 we will also present examples on how different contaminants interact from toxicologic studies of polychlorinated biphenyls performed as part of our albany , new york , superfund basic research program project.imagesfigure 4figure 9