Human arterial smooth muscle cells were obtained from surgically excised tissues and cultured by the explant method. The cultured cells had both active and inactive forms of an angiotensin I forming enzyme. About a five-fold increase in the activity was obtained by trypsin treatment. This renin-like enzyme was also found in abundance in the culture media, mostly in an inactive form. Most of the enzyme activity, either before or after the activation, was suppressed by an antibody specific to human renin. The inactive enzyme was activated to some extent also by acidification and by cold exposure. The molecular weight of the inactive enzyme was estimated to be approximately 49,000 by gel filtration. These results suggest that human vascular smooth muscle cells can produce renin and release an inactive form of renin, and can be a potential source of plasma inactive renin under certain conditions such as the anephric state.