Macrophage aggregates (MAs) are believed to be functional equivalents of germinal centers, active in storage of exogenous and endogenous waste products, the immune response, and iron storage and recycling. Numerous studies have shown an increase in their number, size or hemosiderin content in fish collected at contaminated sites. For this reason, MAs have been suggested as potentially sensitive biomarkers of contaminant exposure. Although they are structures observed histologically, it has also been suggested they may be immunotoxicologic biomarkers. To determine possible relationships between MA formation and macrophage function, we examined data from two field studies and one laboratory study. We found a significant correlation between a decreased chemotaxic response of macrophages and the formation of more numerous, smaller MAs in mummichog from a contaminated site. In laboratory-exposures to arsenic, macrophage function appeared to be a more sensitive indicator at the lower levels of dietary arsenic. However, MA appeared to provide a more dose-dependent and comprehensive indicator of toxicity.