Colostrum and milk samples from 60 Holstein–Friesian cows were analysed for concentrations and yields of immunoglobulin G (IgG), β-lactoglobulin (β-lg), α-lactalbumin (α-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, β-lg 14·3±4·6, α-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-γ-globulinaemia was found in 18·3% of the cows. The concentrations of IgG, β-lg and BSA dropped abruptly in subsequent milkings and α-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.