A solid-phase radioimmuniassay using antibody-coated tubes and 125I-labelled progesterone is described for determining concentrations of progesterone in unextracted bovine plasma. Samples were assayed for progesterone by both an aqueous-phase assay (X) using extracted plasma and the solid-phase assay (Y). The estimates of the concentration of progesterone using these two methods were highly correlated (r = 0.93) and the relationship between them was best described by the equation Y = 1.15X − 0.09. The slope and intercept of this equation were not significantly different from one and zero, respectively (P < 0.001). The technique is sufficiently sensitive to measure progesterone in 0.05 mL of plasma, is specific for progesterone, and has intra- and inter-assay coefficients of variation of 4.7 and 16.7%, respectively.