The development of a practical competitive protein-binding assay for plasma progesterone in cattle is described. With an intra-assay coefficient of variation of 5,46% and an interassay coefficient of variation of 14,25%, the method is sufficiently accurate and sensitive for practical purposes, and for use in routines and surveys. The statistical level of sensitivity was found to be in the region of 0,25 ng/mℓ. based on the confidence limits of zero dose and 0,25 ng/mℓ, with the practical sensitivity level at 0,50 ng/mℓ. Method and reagent blanks were found to be negligible. The specificity of the assay is based entirely on the partial specificity of the petroleum ether used for the extraction of progesterone (87,5% extraction, n=141).