SummaryThe effect of corticosteroid‐induced parturition on transfer of immunoglobulin into mammary secretion of cows has been studied. Seven nonlactating cows in late gestation were injected intramuscularly with a long‐acting synthetic glucocorticoid and the concentrations of IgG1, and IgG2, were followed in blood serum and mammary secretion which were collected before and after parturition.Within 1 week of injection the udder became distended with secretion identical in appearance to mature milk. In contrast to the situation in normal cows approaching parturition, both the concentration of IgG1, in secretion and the selective index of IgG1, decreased after the glucocorticoid injection. A feature of the results was the absence in treated cows of the characteristic decrease in serum concentration of IgG1, just before parturition.It is concluded that glucocorticoid treatment of cows in late pregnancy results in a decreased availability of colostral immunoglobulin to the newborn calf.