The carotene and immunoglobulin concentrations in colostra and milks of 6 Jersey, 4 Friesian and 6 Jersey x Friesian cows were determined during the first 21 d of lactation. Carotene concentrations of between 50 and 300ug/g milk fat were found at first milking and these declined to normal concentrations, in the range 9-21ug/g milk fat, by the eighth to tenth day of lactation. Immunoglobulin concentrations at first milking were in the range 4-40g/l milk and decreased to normal concentrations, in the range 0.3-0.5g/l, by the fifth to seventh day of lactation. For both substances the data from each cow's milk fitted an exponential decay model. The fractional rates of decrease in concentration of carotene were 0.19-0.57/d and of immunoglobulin were 0.40-0.73/d. Breed-group differences were observed in both carotene and immunoglobulin concentrations in milks after the fourteenth day of lactation. In Jersey milks, the average carotene and immunoglobulin concentrations were respectively 18.5 ug/g milk fat and 0.32g/l of milk, while in Friesian milks the concentrations were respectively 11.4 ug/g milk fat and 0.46 g/l milk. There was also a difference between breed groups in the daily rates of decrease of the immunoglobulin concentrations. The average rate of decrease for the Friesians was about 1.5 times the rate for the Jerseys.