The effects of single polychlorinated biphenyls (PCB) on isolated tubuli seminiferi of the rat were studied. Freshly isolated rat tubuli seminiferi were prepared according to their transillumination pattern, i.e. dark or pale. Tubuli seminiferi with the dark pattern included stages II to VIII and tubuli with the pale pattern represented stages IX to XIV and stage I of the seminiferous cycle. Afterwards, tubuli seminiferi were exposed to single polychlorinated biphenyls for 5 or 24 h in vitro. PCB 126, PCB 77, and PCB 118 were used in final parts per billion (p.p.b.) concentrations as determined by quantitative PCB analysis. Eventually, the specimens were plastic embedded, cut into semithin sections, stained, and morphology was evaluated by light microscopy. Single PCB congeners induced morphological alterations in cultivated rat tubuli seminiferi in a time- and stage-dependent manner. Effects comprised loosened intercellular contacts between germ cells and Sertoli cells as well as cellular fragmentation in the layer of round spermatids. Early spermiogenesis seems particularly susceptible to single PCB congeners in concentration of background magnitude. The target cell has still to be discovered.