The American lobster [Homarus americanus (Edwards, 1837)] is an omnivore decapod with generalist and opportunistic feeding habits, in which gastric fluid cysteine and aspartic peptidases are the main proteolytic enzymes. Using in vitro assays, the network of concerted acting peptidases belonging to aspartic and cysteine catalytic mechanisms that hydrolyze bovine serum albumin (BSA), hemoglobin, and actin was elucidated. The role of each class of peptidases in protein digestion was evaluated by monitoring the hydrolysis of each substrate in the presence of single or mixed specific peptidase inhibitors. Peptidases of the gastric fluid can hydrolyze BSA, actin, and hemoglobin. Hemoglobin was hydrolyzed by cysteine and aspartic peptidases but not by serine ones. Results suggest that hydrolases act synergistically. Hydrolysis of BSA and actin is carried out by a multienzyme network mechanism. Although cysteine peptidases hydrolyze about 50% of BSA and actin, aspartic peptidases are required to fulfill the task; when aspartic and cysteine peptidase activity is absent, no hydrolysis of the substrates happens at all. Serine peptidases seem not to contribute to the analyzed substrates hydrolysis at the conditions of the assay.