PurifiedAlternaria alternataaltertoxins I, II, and III were evaluated for comparative cytotoxicity and ability to inhibit gap junction communication in the Chinese hamster lung metabolic cooperation assay. The noncytotoxic test range for each altertoxin was determined for the metabolic communication assays: altertoxin I, 1, 2, 3, 4, 5 μg/ml; altertoxin II, 0.02, 0.008, 0.006, 0.004, 0.002, 0.0008 μg/ml; and altertoxin III, 0.2, 0.1, 0.08, 0.06, 0.04 μg/ml. Altertoxin II was the most cytoxic in the V79 system, followed by altertoxins III and I. The last cytotoxic of the three, altertoxin I, weakly disrupted metabolic communication at two concentrations (4 and 5 μg/ml). Altertoxins III and II did not significantly inhibit gap junction communication more than the weak tumor promoter 4‐O‐methyl ether tetradecanoylphorbol 13‐acetate.