Hemocyanin from the terrestrial snail Levantina hierosolima (Gastropoda, Pulmonata) was studied. A Mw of 10.4 X 106 was determined for the native 100S molecule by sedimentation equilibrium. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave one band with a mobility corresponding to a Mw of 360000. The molecular weight of the polypeptide chain was determined to be 334000 by sedimentation equilibrium in 6 M guanidine hydrochloride and 0.1 M 2-mercaptoethanol. Analysis of the copper content gave 0.229%, corresponding to a minimal weight per two copper atoms of 55 000. We conclude that a 100S molecule of Levantina hemocyanin is composed of 30 polypeptide chains, each of which contains six binuclear copper centers for binding oxygen. These findings constitute a fundamental departure from the 20 polypeptide chain eight functional unit model currently accepted for gastropod 100S hemocyanin.