Decapod crustaceans are a valuable seafood product and although their price is typically gauged by their overall size, the quality of the product (organ/muscle mass and moisture content) is also important. The current study investigated the use of the serum protein concentration (determined from the refractive index) as an indicator of the quality and physiological condition of the adult lobster Homarus americanus (Milne-Edwards, 1837). Lobsters were maintained in a variety of feeding regimes and environmental conditions for 6 mo in both the laboratory and in the field. A hand-held refractometer was used to determine serum protein concentrations indirectly. At the end of the experimental period, the lobsters were killed and the hepatopancreas and heart mass, edible meat content, and moisture content of the hepatopancreas and muscle tissue were measured. In each case, there was a significant correlation with serum protein concentration. Serum protein concentration increased linearly with hepatopancreas mass, heart mass, and edible meat content. There was a concomitant decrease in serum protein concentration with increasing moisture content of the hepatopancreas and muscle tissue. Food intake had a significant effect, with higher serum protein concentrations and thus larger organs, in animals that were maintained on a high-feeding frequency regime. The serum protein concentration also changed during the molt cycle, reaching its highest levels during the premolt stage followed by a sharp decrease after the lobsters had molted. The serum protein concentration increased slowly thereafter and, 12 wk after molting, had reached levels that were similar to those measured at the start of the experiment. This rapid and noninvasive method has the potential to be a valuable tool for determining quality and physiological status of commercially important decapod crustaceans.