In order to gain a deeper insight into the role of 5 alpha-reductase in the growth of beards in men, we studied some kinetic properties of the enzyme in cell homogenates of cultured human dermal papilla cells from beard and occipital scalp hair. When cell homogenates were incubated with [3H]-testosterone, the 5 alpha-reductase of beard dermal papilla cells exhibited an optimum activity at pH 5.5, whereas the enzyme of dermal papilla cells from occipital scalp hair showed a broad and low plateau between pH 6.0 and 9.0, without a sharp peak. The apparent Michaelis constant of 5 alpha-reductase was 3.3 x 10(-7) M in dermal papilla cells from beard and 2.4 x 10(-5) M in those cells from occipital scalp hair. The apparent Km of 5 alpha-reductase for NADPH was 2.8 x 10(-5) M and 7.6 x 10(-4) M in beard and occipital scalp hair dermal papilla cells, respectively. There were no significant differences in the substrate specificity between these two types of cells. The 5 alpha-reductase activity was recovered mainly in the nuclear fraction of beard dermal papilla cells. By contrast, it was widely distributed among the individual subcellular fractions of dermal papilla cells from occipital scalp hair. These results strongly suggest that these two kinds of dermal papilla cells have different types of 5 alpha-reductase, and that the enzyme in beard dermal papilla cells is similar in characteristics to that in the androgen target organs such as prostate.