Based upon our initial observations that 1,1,1-trichloro-2,2-bis(p-chlorophenyl)-ethane (p,p'-DDT) induces a concentration-dependent increase in 3T3-L1 adipocyte differentiation, the mechanism of the p,p'-DDT-induced adipocyte differentiation was studied, using 3T3-L1 and 3T3-F442A cells. Since, it is known that the differentiation of the 3T3-L1 adipocyte cell line involves the induction of the transcription factors CCAAT enhancer binding protein beta (C/EBPbeta), peroxisome proliferator-activated receptor gamma (PPARgamma), and C/EBPalpha, the possible role of these factors in p,p'-DDT-induced adipocyte differentiation had to be examined. It was found that p,p'-DDT-treated 3T3-L1 cells showed a concentration-dependent increase in the nuclear levels of both PPARgamma and C/EBPalpha protein. On the other hand, treatment with p,p'-DDT (20 microM) did not affect the expression pattern of C/EBPbeta protein during differentiation. Gel shift analysis of nuclear proteins for binding to the C/EBP recognition site of DNA showed an increase in binding activity at day 2 of differentiation in p,p'-DDT-treated cells. Supershift analysis revealed that this rise was caused mainly by a dramatic increase in the abundance of the C/EBPalpha-DNA complex. Similar increases were observed at days 4 and 7 after the induction of differentiation. Tumor necrosis factor alpha induced a strong inhibition of adipocyte differentiation, which was reversed by co-treatment with troglitazone, an activator of PPARgamma. p,p'-DDT was unable to reverse the inhibitory effect of tumor necrosis factor alpha on adipocyte differentiation in 3T3-L1 cells. 3T3-F442A is another preadipocyte cell line that can be induced to differentiate into adipocytes in the presence of insulin and fetal bovine serum. p,p'-DDT (20 microM) induced an alteration in the morphology of these cells at day 2 after the induction of differentiation. These cells however, were unable to become fully differentiated adipocytes. These data showed, therefore, the ability of p,p'-DDT to alter the differentiation process of adipocyte cell lines through the modification of transcription factors regulating this event.