We investigated the effect of the tumor necrosis factor-alpha (TNF alpha) on the differentiation of human adipocyte precursor cells in primary culture. Adipocyte precursors convert into fat cells within 12-16 days in a chemically defined, hormone-supplemented medium. Exposure of cultured preadipocytes to TNF alpha resulted in a dose- and time-dependent decrease in the number of developing fat cells and the activity of glycerol-3-phosphate dehydrogenase (GPDH), an established marker of adipocyte differentiation. A 24-h incubation with TNF alpha at a concentration of 5 nM suppressed GPDH activity by 55% compared to that in control cultures. Continuous exposure of the cells to TNF alpha completely blocked expression of the adipocyte phenotype and GPDH activity. The inhibitory action of TNF alpha was not associated with a change in cell number, as assessed by cell counting. The addition of 5 nM TNF alpha for 24 h to newly developed fat cells caused a rapid reduction of GPDH activity by approximately 50%. A 14-day exposure of differentiated cells to TNF alpha was followed by complete suppression of GPDH and a marked delipidation of the cells, including morphological changes, leading to the development of long spindle-shaped cytoplasmatic extensions. These results clearly demonstrate that TNF alpha inhibits the differentiation of human adipocyte precursor cells and, in addition, promotes the delipidation of mature fat cells. It is suggested that TNF alpha may be involved in the physiological control of human adipose tissue cellularity and function.