Hemolymph plasma of the snail Helix aspersa which inhibits maturation and reproduction of its mantle cavity-inhabiting nematode, Rhabditis maupasi, was separated biochemically for the active proteinaceous component.Isolation of the active inhibitor was performed using ion-exchange chromatography in combination with subsequent gel filtration. The isolated peaks were bioassayed in vitro on nematode larvae. The fractions harboring inhibitory protein suppressed larval growth and adult reproduction in vitro.The isolated fraction was purified by gel filtration and characterized on the basis of a single band on starch zone electrophoresis and positive reaction only with folin-phenol reagent.