Measurement of progesterone concentration in blubber was developed as a method to detect pregnancy in minke whales. Progesterone was extracted and quantified from blubber samples of minke whale carcasses by radioimmunoassay. Results showed a highly significant difference (almost 60‐fold) between blubber progesterone concentrations of anatomically determined pregnant females versus non‐pregnant female or male carcasses. The results of the study suggest that the blubber progesterone concentrations might be used to determine pregnancy status in free‐ranging whales.