A solid-phase enzymeimmunoassay (EIA) was developed for progesterone, based on horse radish peroxidase as the enzyme-label and Dasp (sheep anti-rabbit Ig, covalently linked to cellulose) for separation of antibody-bound and free hormone. The validity of the assay was substantiated in accordance with criteria for sensitivity, precision, accuracy and specificity; it is simple to perform and can be done in one day. Milk samples were taken on the day of A.I. (day 0) and on day 21 from 60 dairy cows which belonged to an artificial insemination (A.I.) station in the central part of Thailand. Conception rate (C.R.) after first A.I., as diagnosed by the milk-progesterone test, was 31.7 per cent. Based on rectal exploration on day 84-93 and on 'non-return' at 85 days. C.R. was 20.0 per cent and 23.3 per cent, respectively. All animals except one showed non-luteal phase progesterone levels at the time of A.I. It is concluded that low C.R. in these cows is not due to inseminations during the luteal phase of the oestrous cycle.