The results described here imply the existence of a protein polymorphism in earthworms. Using analytical isoelectric focusing (IEF) the natural hemolysin of the coelomic fluid of Eisenia fetida andrei appears to be constituted by 4 lipoproteins or lipoprotein-bound substances, referred to as isoforms, which are characterized by their different isoelectric points, ranging from 5.90 to 6.30. Each isoform possesses a natural lytic activity against sheep erythrocytes. All animals possess either 2 or 3 isoforms, with one isoform (pH = 6.00) invariably present. The polymorphism determined here is based on the combinations of the 3 other isoforms. Among all the populations tested, only 6 different hemolytic patterns were found. A presumptive genotype based on the hemolytic patterns of some F1 and F2 descendants is proposed. © 1979 Pergamon Press Ltd. All rights reserved.