Colostrum and milk samples from 60 Holstein-Friesian cows were analysed for concentrations and yields of immunoglobulin G (IgG), beta-lactoglobulin (beta-lg), alpha-lactalbumin (alpha-la) and serum albumin (BSA) throughout the first 16 milkings post partum (8 d of lactation) using a single radial immunodiffusion assay. Concentrations (mg/ml, means +/- SD) at first milking were IgG 59.8 +/- 28.5, beta-lg 14.3 +/- 4.6, alpha-la 2.04 +/- 0.6, BSA 1.21 +/- 0.44. Large variations were recorded for IgG concentrations (15.3-176.2 mg/ml) and yields (0.2-925 g). Cows in their first lactation produced significantly lower concentrations and yields of colostral IgG than cows in later lactations. A colostral yield of IgG below the 100 g required to prevent calf hypo-gamma-globulinaemia was found in 18.3% of the cows. The concentrations of IgG, beta-lg and BSA dropped abruptly in subsequent milkings and alpha-la concentration decreased slowly. The mean IgG concentration was < 2 mg/ml after eight milkings and < 1 mg/ml after fifteen milkings. However, IgG concentration did not differ significantly, at the 1% level, during milkings 11-15. The results were tabulated to make it possible to calculate the excess of whey proteins that would be obtained if early milks were illegally added to the milk supply.