Exposure of young striped bass Morone saxatilis (weight, 8.5–34 g) to a mixture of arsenic, cadmium, copper, lead, and selenium at 4 and 10 times the average environmental concentrations of 1–3 μg/L protected the fish from experimental infection with Flexibacter columnaris, the causal organism of columnaris disease. In four trials, all striped bass died within 7 d after a 2-min exposure to 5 × 106F. columnaris cells in untreated water. In contrast, no fish died after a single day's exposure to the metal mixture followed by infection with F. columnaris and a second exposure to the metals for seven more days. When striped bass were exposed 5 d to individual metals, copper protected against infection and cadmium offered marginal protection but was slightly toxic after 2 d exposure. Arsenic increased susceptibility to infection, and lead and selenium were without an apparent effect.