Seminal fluid represents a milieu enabling spermatozoa to break the ovum membrane and suppress its immune response and, at the same time. to protect male germ cells against infects. Among constituents of the seminal fluid various steroids, including dehydroepiandrosterone (DHEA) and its sulphate. Merl detected. With respect to immunomodulatory and antioxidative properties of the latter steroids and its 7-hydroxylated metabolites. believed to be at least in some instances the locally active species, their presence in seminal fluid is of particular interest. Here for the first time unconjugated 3 beta ,7 alpha -dihydroxy-5-androsten-17-one (7 alpha -OH-DHEA) and its 7 beta -hydroxyisomer have been detected and quantified in semen. Eight semen samples were extracted with diethyl ether and Following evaporation and solvent partition both isomers were detected by gas chromatography mass fragmentometry using the ions miz 358 and 343 for quantification. Another portion was separated by HPLC and in the fractions corresponding to 7-OH-DHEA isomers the steroids were measured by recently developed specific radioimmunoassays (RIA). Mean concentrations of 7-OH-DHEA as measured by RIA amounted 5.75 +/- 1.29 and 5.39 +/- 0.75 nmol/l (mean +/- SEM) for 7 alpha- and 7 beta -OH-DHEA. respectively. (C) 2001 Elsevier Science Ltd. All rights reserved.