A series of immunoassays, enzyme activity tests and growth rate analyses were conducted on the free living polychaete Eurythoe complanata after 21 day exposure to 0.2 mg.L-1 sublethal nominal concentration of Cu2+ (CuSO45H2O). The immunological assessment was further made immediately after 15 and 30 day post-exposure. Immune cytological parameters, coelomocyte viability and differential coelomocyte counts showed significant differences between exposed an non-exposed worms. Coelomocyte responses to antigenic mouse red blood cells (MRBC), such as formation of secretory (SR) and erythrocyte rosettes (ER), and phagocytosis evidenced significant impairments in the immunological defenses related to copper contamination, related to a two fold increase in Cu-carcass tissue concentration (13.70 +/- 1.50 micrograms.g-1 dry wt, n = 6). The normal immune functions were reestablished upon depuration as tissue Cu content approached basal levels (7.40 +/- 1.70 micrograms.g-1 dry wt, n = 6). Neither enzyme activity of coelomic transaminases as indicators of tissue damage, nor growth rate indexes (carcass tissue RNA/DNA and protein/DNA ratios) were affected by copper exposure. In conclusion, the immunoassays yielded highly sensitive results compared to biochemical markers, suggesting the potential usefulness of immune parameters as environmental stress indicators for sediment/aquatic bioassays with the polychaete Eurythoe complanata.