The pyrogenic properties of some C-19 and C-21 steroids were examined by in vitro incubation of human blood leukocytes with serum-buffer solutions of the steroids and injection of the 18-hr supernatants into rabbits. In previous studies this method demonstrated release of leukocyte endogenous pyrogen by etiocholanolone. With two exceptions, steroids known to cause fever in man, such as 11beta-OH etiocholanolone and 3alpha-hydroxy-5beta-pregnane-20-one were also pyrogenic in vitro. All steroids tested which are nonpyrogenic in man, such as androsterone, 3beta-OH etiocholanolone, and 3alpha, 17alpha-dihydroxy-5beta-pregnan-20-one were also nonpyrogenic in vitro. Solubility in aqueous solution did not correlate with pyrogenic capacity. Inhibition of pyrogen release from human leukocytes in vitro by hydrocortisone and estradiol was demonstrated. Hydrocortisone-treated leukocytes released less pyrogen than did normal leukocytes when stimulated either by etiocholanolone or by phagocytosis of heat-killed staphylococci. On the other hand, estradiol-treated blood leukocytes and mononuclear cells showed significant suppression of pyrogen release when phagocytosis, but not etiocholanolone, was used as the stimulus. When blood cells were incubated with progesterone, greater than normal amounts of pyrogen were released following phagocytosis, and the inhibiting effect of estradiol could be partially reversed. Neither estradiol nor hydrocortisone appeared to act on rabbit leukocytes. These studies indicate that a variety of naturally-occurring steroids may alter pyrogen release from leukocytes. Alterations in steroid balance in man may influence normal temperature regulation and contribute to clinical fevers.