Abstract Young male Holstein calves were fed diets containing 40 or 640ppm zinc with 0 or 5ppm aflatoxin for 3 wk. The aflatoxin mixture contained 80.5% B1 and the calves consumed 143mg of B1 over 3 wk. Plasma glutamic-oxaloacetic transaminase and alkaline phosphatase concentrations were increased substantially, and lactic dehydrogenase was reduced in aflatoxin-fed calves. Supplemental zinc partially counteracted the effect of aflatoxin on these enzymes. Hemoglobin, packed cell volume, and total solids in blood plasma were increased in aflatoxin-fed calves, but high dietary zinc had no effect on these blood constituents. Glucose in plasma was reduced in calves receiving aflatoxin. High dietary zinc was only partially effective in protecting against the reduced glucose effect for about 1wk. Total protein, albumin, globulin, ratio of albumin/globulin in blood plasma, and liver lipid were not affected by aflatoxin. Several enzymes and blood constituents are affected by alfatoxin in calves. The protection of zinc against aflatoxicosis appears to be no more than a partial effect.