Objective To investigate the effect of calcipotriol on melanin synthesis by human melanocytes and its possible action mechanism.Methods Primary melanocytes were cultured with various concentrations(10~(-5),10~(-6),10~(-7),10~(-8),10~(-9),10~(-10) mol/L)of calcipotriol for 24 or 48 hours.Subsequently,MTT assay,NaoH assay.Dopa-oxidase assay,Western blot and semiquantitative RT-PCR were used to measure the cell proliferation of,melanin synthesis by.tyrosinase activity,protein and mRNA expression levels in the melanocytes.respectively.Those untreated melanocytes served as the control.Results The calcipotriol between 10~(-9) and 10~(-5) mol/L had no significant effect on the proliferation of cultured melanocytes(P＞0.05).while that of 10~(-9) and 10~(-8) mol/L increased tyrosinase activity by 137%and 123%,and enhanced melanin synthesis by 40.63%and 18.75%,respectively,compamd with untreated melanocytes(both P＜0.05).Moreover,the tyrosinase protein level increased by 270.4%(P＜0.05)in melanocytes treated with calcipotriol at 10~(-9) mol/L for 24 hours.The strongest tyrosinase activity and melanin synthesis was observed in melanocytes treated with calcipotriol of 10~(-9) moI/L.Conclusions The proliferation of melanocytes is unaffected by calcipotriol at 10~(-9) to 10~(-5) mol/L,but it can elevate the expression of tyrosinase protein,enhance tyrosinase activity,and promote melanin synthesis in melanocytes.