Primary ciliary muscle cell cultures derived from human donors (16-91 years) were established and characterized by comparing them with ciliary muscle in tissue sections using immunocytochemical and ultrastructural methods. Monoclonal antibodies against desmin, vimentin, alpha-actinin, smooth muscle (sm) specific alpha-actin and von Willebrand factor were used. In tissue sections of the ciliary body, ciliary muscle cells, vascular muscle cells, pericytes, endothelial cells and fibroblasts stain for vimentin. Both types of muscle cells and the pericytes stain for alpha-sm-actin, but only ciliary muscle cells stain for desmin. For tissue cultures, explants of the meridional and partly the reticular portion of the ciliary muscle were dissected and grown directly or after digestion of the explant with collagenase. Ten primary cell cultures with a typical hill-and-valley growth pattern similar to smooth muscle cells and two with a growth pattern similar to fibroblasts were established. All cultures could be subcultured up to the fifth passage. In fibroblast-like cultures 5-10% of the cells stained for alpha-sm-actin. Staining for desmin was not observed. In smooth muscle-like cultures, all cells stained positive for alpha-sm-actin. Desmin staining was not seen in growing non-confluent smooth muscle-like cultures. In confluent cultures, about 10% of the cells stained positive for desmin, preferentially in areas where the cells had formed hills. No culture stained for von Willebrand factor. Staining for alpha-actinin in smooth muscle-like cultures showed that the dense bands of the myofilaments were arranged in register, similar to the typical ciliary muscle cell morphology seen in tissue sections. Ultrastructurally, the smooth muscle-like cultures showed the typical morphology of cultured smooth muscle cells. We conclude that the smooth muscle-like cultures consist of ciliary muscle cells.