Acute toxicities of oxytetracycline hydrochloride (OTC) and calcein to juvenile striped bassMorone saxatilis(48 ± 5 mm total length) were determined by immersion. Fish were held in test solutions for 6 h, then placed in clean water and observed for 96 h post-exposure. Both OTC and calcein bind to calcium and fluoresce under ultraviolet light, which makes them useful for marking bony structures of fish. Information on toxicity of these chemicals to juvenile striped bass will help establish maximum concentrations that can be used to mark hatchery fish stocked for population enhancement or establishment. The no-observed-effect concentration (NOEL), 96-h LC 10, and LC50 (concentrations lethal to 10 and 50% of test fish) values for the 6-h chemical exposure were 447, 322, and 597 mg/L for OTC and 125, 160, and 240 mg/L for calcein.