Abstract A simple and rapid quantitative method has been developed for the estimation of sperm ecto-SH groups on the basis of their high affinity binding to the mercurial: [ 203Hg] p-chloromercuriphenylsulfonic acid (PCMPS) used as a surface probe. The thiol reagent did not penetrate the sperm plasma membrane, as evidenced by the extremely rapid time course of the binding reaction and undetectable uptake of [ 203Hg]PCMPS by intact goat spermatozoa. The binding reaction was not due to contaminating broken or damaged cells, if any. The method consists of incubating of highly motile goat spermatozoa with PCMPS in a modified Ringer solution at 37 °C for 5 min, agglutination of the labelled cells with polyethyleneimine (100μg/ml) and filtration and washing of the cell suspension through Whatman No. 1 filter discs under mild vacuum. The binding interaction is proportional to cell concentration, specific and saturable at 50 μM PCMPS. The method is capable of estimating free ecto-SH as low as 25 pmoles. Spermatozoa possess 286 ± 61 pmoles of free ecto-SH groups/10 6 cells. Scatchard analysis showed the presence in goat spermatozoa of multiple classes of ecto-SH groups differing in their affinity for PCMPS.