A method is presented for simultaneous determination of submicrogram quantities of testosterone and epitestosterone in human plasma. These steroid alcohols are determined as O-methyldichlorotetrafluoroketals (2,4 MCFK) by electron capture detection. Ten to 20 ml. of plasma, with 4-I4C testosterone added for recovery, is extracted and purification is achieved by thin-layer chromatography (TLC). The hemi-ketals, which are formed by addition of 1, 3-dichloro-l,1,3, 3-tetrafluoroacetone are quantitatively converted to methyl ethers by diazomethane, Plasma testosterone, separated as testosterone-2,4 MCFK was collected from gas chromatograph and identified by its ultraviolet (uv) absorption. The infrared (ir) spectra of testosterone, epitestosterone and their 2,4 MCFK derivatives are reported.