Ultrastructural and some immunophenotypic features of the peritubular myoid cells of testes from normal men and from men with varicocele were studied. The seminiferous tubules were classified into five types (a-e), related to the progressive degree of sclerosis measured as thickening of the lamina propria. In normal testes only type a and b tubules were found, whereas the testes from men with varicocele showed type b-e tubules. Myoid cells in tubule types a and b showed slender cytoplasmic projections with abundant, parallel arranged microfilament bundles and electron-dense bodies. In c tubules, the myoid cells showed the same ultrastructure. The myoid cells of tubules with advanced (type d) or complete (type e) sclerosis showed irregularly outlined nuclei, scant microfilament bundles and absence of electron-dense bodies. Immunostaining of myoid cells with anti-actin antibodies was intense in types a-c tubules and scant in types d and e. Immunostaining with anti-desmin antibodies was intense in tubules types a-d, but the immunoreactive cells in types c and d tubules were irregularly shaped and distributed and were scanty in tubule type e. Immunostaining with anti-vimentin antibodies was weak in types a-c tubules and intense in types d and e tubules. Quantitative studies revealed that, with the progression of sclerosis, the numbers of both actin- and desmin-immunoreactive cells per cross-sectioned tubule, and the surface area occupied by the immunostained portion of these cells, decreases while the number of vimentin-immunoreactive cells and their immunostained surface area increases.