The growing awareness of adverse effects of environmental contamination has lead to scrutiny of various biomarkers as early indicators of these effects. The use of splenic macrophage aggregates (MA) as indicators of fish health and environmental degradation was evaluated using three fish species from four sites. The sites were part of the Environmental Protection Agency's Environmental Monitoring and Assessment Program (EMAP). Water quality, benthic and fish community characteristics, sediment and fish tissue contaminant analysis were all conducted at these sites. In addition, tissues from pinfish (Lagodon rhomboides), spot (Leiostomus xanthurus) and gafftopsail catfish (Bagre marinus) were fished in the field. Histological sections of spleen were evaluated using a numerical rating system and image analysis. The MA number, size, and percent of tissue occupied by MAs of each species was compared among sites and related to either tissue or sediment contaminant levels. Spot MA parameters were all significantly higher at the site with the highest sediment contaminant levels. Catfish accumulated high tissue burdens of most contaminants when compared to other fish collected at the same site. Catfish MA parameters were also better indicators of tissue contaminant levels. The usefulness of MA as indicators of environmental stress is supported by this study. Choice of fish species to evaluate sites is discussed.