Synthetic 2-hydroxyethylmethacrylate copolymer particles (HEMA) can be opsonized in the coelomic fluid of Eisenia foetida earthworms. The incomplete coelomic fluid (i.e. the coelomic fluid after incubation with HEMA particles) exerts a lower level of hemolytic activity compared to complete coelomic fluid. The decreased hemolysis can be compensated by the addition of isolated opsonins. On the other hand, isolated opsonins do not possess direct hemolytic capacity. It can be suggested that at least one of the isolated opsonins is involved in the hemolytic process. These results support the hypothesized cooperation of humoral and cellular mechanisms in earthworm defence.