Adipose precursors isolated from the stromal-vascular fraction of omental and subcutaneous adipose tissue, from defined hyperplastic obese and nonobese human adults were cultured in order to measure and compare replication rates. After multiplication to confluence these cells were also cultured in an enriched viscous suspension medium to optimize the expression of these cells to adipocytes, allowing an estimation of the number of cells having the ability to express an adipocyte phenotype. No difference in replication rate was seen between obese and nonobese donors or when adipocyte precursors from different depots were compared. When cells were allowed to develop fully in the enriched medium, approximately 6.5% of the original inoculated cell population exhibited an adipocyte morphology. Thus, these results suggest that environmental rather than genetic factors may be responsible for the hyperplasia seen in certain massively obese humans. Furthermore, the results indicate that fat-free cells found within the stromal-vascular fraction of adipose tissue have the ability to develop into adipocytes. However, it is suggested that the relatively low yield in obtaining fully differentiated fat cells under these conditions may be due to the heterogeneity of adipose related cells within the original stromal-vascular fraction from which these cultures were initially derived.