Specimens of Spirographis spallanzani (Polychaeta: Sabellidae) were collected in the Gulf of Taranto, Italy, in 1991, and their mucous secretion was examined. The mucus possesses thermolabile biologically active proteins which produce hemolysis in vertebrate erythrocytes. The molecules are ineffective against both unfertilized and fertilized echinoid eggs and spermatozoa. The lytic reaction against rabbit erythrocytes varies as a function of cations, pH and temperature, and displays a fast time-course. No immunological cross-reaction could be demonstrated between hemolysins of S. spallanzani and Eisenia foetida andrei. SDS-PAGE analysis of mucus proteins reacting with sheep erythrocytes revealed that at least three major (16, 22 and 40 kDa) and two minor (29 and 34 kDa) components interact with membrane targets.