A radioimmunoassay for the estimation of progesterone in plasma from the domestic fowl is described. Antibodies were raised in goats to progesterone conjugated at the 11 position of the steroid nucleus to bovine serum albumin. An antiserum of high specificity was obtained which allowed the estimation of progesterone in extracts of plasma without purification. The sensitivity of the assay, which was 25 pg using an antiserum dilution of 1:8000, allows the determination of progesterone in 200 μl plasma. The accuracy and precision of the method were highly satisfactory. Plasma progesterone levels in laying hens ranged from 0.67–9.60 ng ml−1 with a mean value of 3.13 ± 0.37 ng ml−1 but were low (< 0.10–0.67 ng ml−1) in plasma from moulting, ovariectomised and hypophysectomised hens and in plasma of intact and castrated cockerels. Progesterone concentrations in blood obtained from a follicular vein ranged from 12.8–133.1 ng ml−1; these values were 8–50 times higher than in peripheral blood collected concurrently. The results suggest that high concentrations of progesterone found in the blood of laying hens are associated with the ovulatory cycle and that the preovulatory follicle secretes significant amounts of progesterone.