Two four-treatment crossover bioavailability studies were performed in panels of 12 adult male volunteers with eight different commercial prednisone tablets. Plasma samples from the first study were assayed by radioimmunoassay for both prednisone and prednisolone. Plasma samples from the second study were assayed for prednisolone only. Statistical analyses of the data showed significant differences in the rate of appearance of prednisolone in plasma, but not in the amount convened to prednisolone. Some observations are made on the relationships between prednisone and prednisolone concentrations in plasma following oral administration of prednisone .