Recent studies have shown that, in addition to free steroid hormones, those bound to albumin in plasma may also be available to tissues. In this report, the effects of free fatty acids (FFA) on the binding of steroids to albumin were compared for the cases of bovine serum albumin (BSA) and human serum albumin (HSA). The apparent association constant, K(a), was estimated from the changes in the equilibrium partition coefficient of steroids between the aqueous/hexane phases caused by the addition of albumin to the aqueous phase. In the case of BSA, K(a) for progesterone and testosterone increased upon binding of FFA (myristic, palmitic and stearic acid) to BSA and the maximum value of K(a) for these steroids could be attained by 3-4 mol of FFA bound per mol BSA. Furthermore, the elution profiles of gel-filtration chromatography clearly showed that progesterone and testosterone are easily liberated from the steroid/BSA complexes and that FFA potentiates the binding of these steroids to BSA. In the case of HSA, the binding affinities of progesterone and testosterone were not greatly affected by bound FFA. On the other hand, the affinities of ethynylestradiol to both BSA and HSA were unaffected below 4 mol of FFA binding per mol.