Objective To detect the nucleation-promoting potency of retinol binding protein (RBP) in the simulated bile system,and observe the process of cholesterol nucleation and crystallization.Methods Small and synthetic bile systems were synthesized.RBP (RBP group),α1-acid glycoprotein (AAG,AAG group) and albumin ( Alb,Alb group) were added into the Small bile respectively.The derived indices of nucleation time,crystal growth rate and final concentration of crystal were studied by Busch method.Alb ( control group) and RBP (experimental group) were added into the Small and synthetic bile systems respectively.The nucleation time was detected by polarizing microscopy according to the Holan method and the nucleation-promoting potency was calculated according to the Holzbach method. The morphological dynamics of cholesterol nucleation and crystallization were observed under a light microscope.All data were analyzed by using the analysis of variance or the t test.Results The derived indices of nucleation time,crystal growth rate and final concentration of crystal were 0.66,1.29 and 1.01 in the AAG group,and 0.73,1.02,0.95 in the RBP group,respectively.In the Small bile system,the average nucleation time of cholesterol monohydrate crrystals were ( 12.2 ± 1.2 )days in the control group and (8.2 ± 1.5)days in the experimental group,respectively,with a significant difference between the 2 groups (t =2.97,P < 0.05 ).The nucleation activity of RBP was 0.67 in the Small bile system.In the synthetic bile system,the average nucleation time of cholesterol monohydrate crystals were (9.5 ± 1.1 ) days in the control group and (7.5 ± 1.1 )days in the experimental group,respectively,with a significant difference between the 2 groups ( t =2.35,P < 0.05 ).The nucleation activity of RBP was 0.79 in the synthetic bile system.The process of cholesterol nucleation and crystallization was divided into 4 stages,and the split phenomenon appeared in the bile system at day 21.Conclusion RBP is a pro-nucleation factor in simulated bile system. Key words: Cholelithiasis; Cholesterol; Nucleating proteins