The objective of this work is to determine whether increased (but environmentally realistic) temperature, hypoxia, sulfide and ammonium, alone or in combination, can increase susceptibility of lobsters to microbial infection. Lobsters from eastern Long Island Sound (LIS) were injected with Aerococcus viridans var homari, a pathogen that causes a disease known as gaffkemia. Injected animals (and controls) were placed in a flow-through seawater-system with mechanisms for control of temperature, dissolved oxygen, sulfide and ammonium levels as well as disinfection of effluent. Exposure variables included 0.1 mL injections of A. viridans at doses of 1 × 10³ and 1 × 10⁶; dissolved oxygen at 2.5–6.3 mg L⁻¹; sulfide at 0–21 μM; ammonium at 0–80 μM and temperatures at 14.5 and 19.5°C. The criterion for stressor effect was the time at 50% survival in each set of 15 to 22 lobsters per treatment variable. Also, at regular time intervals, lobster hemolymph and hepatopancreas tissues were analyzed for bacterial levels. When lobsters were held under normoxic conditions at 19.5°C, rates of death from gaffkemia were accelerated in the presence of sulfide above 4 μ M. When lobsters were subjected to moderate hypoxia (3 mg L⁻¹), death rates were accelerated regardless of the presence of sulfide. Exposure to ammonium up to a level of 80 μ M had no effect on death rates. Bacterial counts were similar in lobsters regardless of exposure to stressors. The geometric median count in the hemolymph for all lobsters infected beyond 3 days was 7.7 × 10⁸ ml⁻¹ (maximum raw value 1.6 × 10⁹ ml⁻¹) and that for hepatopancreas was 7.7 × 10⁷ g⁻¹ (maximum raw value 1.1 × 10⁹ g⁻¹). Our work showed that, at 19.5°C (a peak, summer, bottom-water temperature routinely found in Long Island Sound), relatively moderate levels of hypoxia as well as sulfide in the absence of hypoxia may accelerate deaths in lobsters that are infected with a pathogenic bacterium. Because eutrophication may lead to hypoxia and increased sulfide levels, policies that reduce eutrophication may improve lobster health.