Wild-caught female harbor seals (Phoca vitulina) were classified as sexually mature or immature on the basis of standard body length ( 125 cm mature) and plasma progesterone concentrations measured using an enzyme-linked immunosorbent assay (ELISA), a technique usable in the field. Sexually mature females were classified as pregnant or non-pregnant on the basis of their plasma progesterone concentrations. Of 28 wild mature female harbor seals caught in the Moray Firth, N.E. Scotland, between the end of February and the end of May, 79% had plasma progesterone concentrations greater than 60 nmol liter−1, the lowest plasma progesterone concentration measured in one of eight females later observed with a pup, and were diagnosed as pregnant. A linear discriminant function, calculated to provide a method of distinguishing pregnant and non-pregnant females, predicted 100% of non-pregnant females and 95.8% of pregnant females using plasma progesterone concentration, standard length, and month of capture as parameters. Plasma progesterone concentrations were less than 30 nmol liter−1 in all mature and immature males and immature females. In mature females plasma progesterone concentrations ranged from 0-318 nmol liter−1.