Three experiments involving 52 baby pigs were conducted to determine the minimum copper requirement of baby pigs fed purified diets. Diets were supplemented with anhydrous cupric sulfate to yield the following copper concentrations (ppm, by analysis) when the three experiments were combined: 0.6, 0.9, 1.3, 1.9, 2.0, 2.8, 3.2, 4.0, 4.9, 5.6 and 9.3. Parameters examined include weight gain, hematocrit, hemoglobin concentration, mean corpuscular hemoglobin concentration, plasma ceruloplasmin activity, plasma copper concentration, copper balance, brain and erythrocyte superoxide dismutase activity, copper concentration of liver, kidney, spleen, heart, brain, femur and hair, liver ferritin-iron and total iron concentration, strength characteristics of the femur, and gross and histological appearance at necropsy. Weight gains were subnormal at dietary copper concentrations below 1.9 ppm; plasma ceruloplasmin activities, and plasma and tissue copper concentrations were depressed at dietary copper levels below 2.8 ppm. Bone histopathology was evident at dietary copper levels below 3.2 ppm, and copper balance was low at dietary copper levels below 4.9 ppm. Some evidence of anemia was present at dietary copper levels below 5.6 ppm. Under the conditions of this study, the copper requirement of the baby pig fed a purified diet was judged to be approximately 5.6 ppm (6 ppm copper, dry basis).