The present study demonstrates that molluscan immunocytes are able to produce a chemical bacteriocidal substance which can be indirectly identified as nitric oxide (NO). The cells were analyzed in vitro on slides using computer-assisted microscopic image analysis to detect changes in cell conformation as well as to quantify the number of bacteria present. Sodium nitroprusside yields NO in solution causing bacterial clumping. The same phenomenon occurs in the presence of invertebrate immunocytes. Escherichia coli lipopolysaccharide also increases the number of bacteria found around the immunocytes, but this effect is selectively prevented by the addition of inhibitors of nitric oxide synthase, suggesting that this bacterial clumping is caused by the cells liberating NO. Interestingly the cells presumably producing NO maintain a round morphology. These findings suggest that immunocytes are able to kill bacteria by two mechanisms, i.e., phagocytosis and NO production.