Abstract： Objective To investigate the effect of heat treatment combined with narrow band ultraviolet B (NB-UVB) on cultured normal human melanocytes in vitro.Methods Melanocytes were isolated from the foreskin of normal human,cullured in vitro,and irradiated with NB-UVB of different doses (20,30,50,70,90,120 and 180 mJ/cm2).Then,MTT assay was performed to evaluate the proliferation and activity of melanocytes to determine the optimal dose of UVB for the next experiment.Melanocytes were classified into 3 groups to be treated with heat at 42 ℃ for 1 hour (heat group),irradiated with UVB at 50 mJ/cm2 (UVB group),or irradiated with UVB at 50 mJ/cm2 followed by heat treatment at 42 ℃ for 1 hour (combination group),daily for 3 successive days; those receiving no treatment served as the control.After 24-hour culture following the last treatment,tyrosinase activity was evaluated with L-dopa as the substrate,melanin content was detected by NaOH assay,and cell cycle stages were determined by flow cytometry.Results NB-UVB irradiation decreased the viability of melanocytes in a dose-dependent manner,and the optimum dose of UVB was 50 mJ/cm2.The tyrosinase activity of melanocytes was 0.244 ± 0.018 and 0.310 ± 0.015 respectively in the UVB group and combination group,and increased by 3.8％ (P ＜ 0.05) and 31.9％ (P ＜ 0.05) respectively compared with the control group (0.235 ± 0.018); the melanin content was 0.201 ± 0.016 and 0.286 ± 0.019,respectively in the UVB group and combination group,and increased by 17.5％ (P ＜ 0.05 ) and 67.3％ (P ＜ 0.05) compared with the control group (0.171 ± 0.016).In comparison with the control group,the percentage of melanocytes in G1 phase was decreased by 23.94％ in the UVB group (P＜ 0.05) and 33.51％ in the combination group (P ＜ 0.05),while that in S phase and G2 phase increased by 15.35％ (P ＜ 0.05 ) and 11.93％ (P ＜ 0.05),respectively in the UVB group,and 17.76％ (P ＞ 0.05) and 16.08％ (P ＞ 0.05),respectively in the heat group.Conclusion Heat treatment and NB-UVB can synergistically enhance the tyrosinase activity and accelerate melanogenesis,proliferation and differentiation,of melanocytes.