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of the hemolytic agent. Although the concentrations of LPS used for Figure 1B are high, the actual concentration of LPS at the surface of the bacterium is far higher than the solution concentrations used in this trial. The possibility that the hemolytic agent does bind to gram-negative bacteria deserves further investigation. The ability to destroy foreign cells that come in contact with the blood is an important defense strategy for a variety of animals. In mammals, the cytolysis of foreign cells is conducted by the complement system, a multi-component ensemble of plasma proteins whose membrane attack elements are activated by a proteolytic cascade that, itself, is initiated by a variety of stimuli indicative of parasitic invasion (7). The complement system is found only in the deurostomate animals (i.e., the echinoderms and the chordates) and is absent from protostomate animals (8, 9). In the latter, the relatively few cytolytic systems that have been characterized are less complex than the vertebrate complement system, with some the province of a single protein that both recognizes and binds to the foreign cell and mediates its cytolytic destruction (5, 10). Only a few cytolytic systems have been reported in the plasma of molluscs (11, 12), and we have not found any reports for gastropods. The systems reported from bivalves (11, 12) are Ca+2dependent, suggesting that they are based on different effector molecules than the system described here from Busycon. Supported by Grant No. MCB-97-26771 from the National Science Foundation.