Human promyelocytic leukemia cell line, HL-60, is known to proliferate exponentially in a serum-free synthetic medium supplemented with insulin and transferrin (BREITMAN, T.R. et al. (1980). Exp.Cell Res., 126 : 494-498). Among four HL-60 cell lines tested in this medium (serum-free medium), however, a cell line, HL-60/Biken ceased to proliferate after two days culture and most of the cells died within a week. Addition of purified serum lipoprotein (LDL or HDL)to the serum-free medium almost completely restored the proliferating activity of the cells. Total lipids extracted from the lipoproteins could replace the lipoproteins in promoting cell proliferation. Among various lipid components of the lipoproteins, only cholesterol showed a high stimulatory effect on cell proliferation, whereas other lipids tested were ineffective, except for sphingomyelin, cerebroside, and phosphatidic acid which showed limited stimulatory effects. As for the intermediates of cholester l biosynthesis, desmosterol was also effective, whereas lanosterol was rather inhibitory. Chromatographic analyses of the lipids synthesized by HL-60/Biken cells and wild type cells (HL-60/RCB) cultured in serum-free medium, clearly demonstrated that, in HL-60/Biken cells, cholesterol synthesis was almost completely blocked and lanosterol was accumulated 10-fold that in wild type HL-60/RCB cells. All of these results indicate that HL-60/Biken is a variant cell line defective in cholesterol synthesis in the process synthesizing desmosterol from lanosterol. The variant HL-60 cells showed a marked resistance to cell differentiation along both monocytic and granulocytic pathways when compared with wild type HL-60 cells. The cell line may be useful for the study of the role of cholesterol in cell differentiation.