ABSTRACT: Phagocytic responses in circulating hemocytes of the lobster Homarus americanus were measured before and after treatment of lobsters with 2 different immunogens: (1) lipolysaccharide (LPS) or endotoxin from a non-pathogenic Pseudomonas perolens, and (2) a vancomycin/live Gram-positive pathogen (Aerococcus viridans [var.] homari) combination, essentially attenuated cells, shown previously to induce a high degree of resistance to this pathogen. The responses elicited by each of the immunogens were markedly different. Hemocytes drawn from LPS-treated lobsters showed significant, largely non-specific, increases in phagocytic responses over baseline values against sheep red blood cells and an array of test bacteria, with the notable exception of the pathogen. In marked contrast, induction with the vancomycin/live pathogen combination resulted in highly significant and specific increases in phagocytic responses to the pathogen and to the related, (but avirulent) strains of the pathogen, as well as inducing in the lobsters the usual high degree of resistance to the pathogen. These results suggest that quantitative and qualitative variations in phagocytic and resistance levels induced in at least 1 crustacean genus are determined largely by the particular characteristics of the immunogen.