A serologic response was observed in penaeid shrimp which had been exposed to killed vibrio cells. Forty-eight hours after exposure a bactericidin and an agglutinin were observed in the hemolymph of the exposed shrimp. The agglutinin and bactericidin disappeared approximately 120 hours after exposure.  At this point, however, phagocytic activity increased and remained at a relatively high level for 2 weeks.  Pond studies suggest that when challenged shrimp are protected for at least 6 weeks after exposure to the bacterin. Preliminary characterization studies of the hemolymph components suggest it to be a lectin. A theory is proposed that lectin production is a primary adaptive response of shrimp and perhaps other decapods to "antigenic stimulus" and that lectins or closely related components prime certain types of hemocytes for phagocytic activity.