The toxicity of a polychlorinated biphenyl (PCB) mixture (Aroclor® 1254) to mink (Mustela vison) was evaluated through three dietary LC50 tests. These tests were designed to assess the effects of age of the test animals, composition of the diet, and season and year on the results of 28-day dietary LC50 tests. A dose-dependent weight loss was noted for animals in all tests and similar dietary LC50s were calculated for all three trials, both at the end of the 28-day exposure period (83, 84, and 79 ppm) and after a 7-day withdrawal period (58, 47, and 49 ppm). Based on all test criteria, age, dietary composition, season, and year had little effect on the outcome of the LC50 tests. The withdrawal period was important in determining the toxicity of Aroclor 1254 and should be considered in assessing the toxicity of other slow-acting, lipophilic compounds.