The neutral red uptake (NRU) cell viability assay was adapted for use with lobster Homarus gammarus (Linnaeus, 1758) granulocytes cultured in vitro. The assay was more sensitive than the conventional trypan blue exclusion assay and facilitated a higher sample throughput than subjective microscope-based assessments of cell viability. The NRU assay was demonstrated to have a linear response from 470 to at least 126 000 cells cm−2. It was used to investigate the acute cytotoxicity of three commercial and two candidate crustacean aquaculture immunostimulants on lobster granulocytes. All five stimulants had a cytotoxic action on the granulocytes and the toxic dose for some of these stimulants was found to be below their commercially prescribed dose. The long term energetic cost of the use of these stimulants and the concomitant potential for a reduction in growth rate of cultured decapod crustaceans, which is fundamental to the success of commercial aquaculture, is identified and discussed.