Protein kinase activity of intact, motile sperm was assessed by measuring the transfer of the terminal phosphate from [32P]ATP to tricholoroacetic acid (TCA)-precipitable casein. The action of TEST (TES and Tris) yolk buffer (TYB) treatment on phosphorylation of sperm and TYB proteins was studied by detecting labelled phosphoproteins by autoradiography of polyacrylamide gel electrophoresis (PAGE). Results demonstrate that intact, forward-motile sperm have cell surface protein kinase activities. Although the difference between the kinase activity of freshly ejaculated sperm and sperm incubated in TYB was not significant, the protein phosphorylation during incubation in TYB showed that: (i) specific sperm surface proteins were phosphorylated to different degrees during the course of treatment; (ii) TYB proteins were phosphorylated by sperm during incubation; (iii) solubilised [32P]-labelled surface proteins were similar in molecular weight to TYB-labelled proteins. Taking into account that specific proteins on the human sperm surface undergo phosphorylation during incubation in TYB and that the sperm enzyme also acts specifically on some TYB proteins that become attached to the surface of the sperm, working hypotheses are proposed that suggest some correlation between the preservation of semen in TYB and the phosphorylation of proteins by intact human sperm.