We examined some biological activities of a 200‐kDa glyco‐protein, referred to asParacentrotus lividusvitellogenin, contained both in the coelomic fluid and in a subpopulation of coelomocytes called ≪colourless spherula cells≫. Cell‐free coelomic fluid, coelomocyte lysate and supernatant obtained after coelomocyte washings were assayed for hemagglutinating activity. All samples agglutinated rabbit erythrocytes in a calcium‐dependent way. The comparison between the electrophoretic patterns of erythrocyte lysates, before and after incubation with the coelomic fluid, revealed that a 200‐kDa band was obtained from membranes of agglutinated erythrocytes. In addition, polyclonal antibodies against 200‐kDa gly‐coprotein from sea urchin embryos used in Western blot analysis recognized the 200‐kDa glycoprotein only when erythrocyte lysates previously incubated with coelomic fluids were assayed. These results suggest that the 200‐kDa glycoprotein could be an agglutinin, present in the coelomic fluid and released by coelomocytes during stress conditions.