In the present work, the cellular defense reaction of the spiny lobster Panulirus interruptus was studied after being inoculated with the sulfated polysaccharide carrageenan. Three in vivo experiments were undertaken, where 0.2 mg mL–1 carrageenan were administered to adult lobsters and hemolymph samples were taken at different times (0, 1, 3, 6, 12, 24, 36 and 48 h). The total hemocyte count and the types, such as hyalocytes, granulocytes and large granule granulocytes, were recorded. After 24 h, the total hemocyte count in the control group was lower than that observed in the experimental group. During the in vitro experiments, the percentage of phagocytosis by lobster hemocytes was studied using the Gram-negative bacteria Escherichia coli. A phagocytosis of 7.6% to 31.9% was observed using various carrageenan concentrations (0.0%, 0.05%, 0.1% and 0.5%) in the presence of lobster serum, whereas in the reference treatment, E. coli incubated in artificial lobster hemocyte medium showed lower phagocytic activity (6.2% to 22.2%) at the same carrageenan concentrations. It is concluded that lobsters in the presence of a sulfated glycan had a nonspecific response, with the resulting increase in cellular level and indirect phagocytosis stimulation.