When confluent cultures of cloned mouse 3T3-L1 cells were differentiated to adipocytes by three days of treatment with a combination of 0.5 microM dexamethasone and 0.5 mM 1-methyl-3-isobutylxanthine, the S100 protein content in the cells increased markedly, as determined by a sensitive immunoassay system. The S100 protein induced in the cell was the alpha alpha form (S100ao), which is the predominant form of S100 protein in mouse adipose tissue. The S100ao concentration in preadipocytes was about 1-3 ng/mg protein, while the concentration in differentiated adipocytes was 60-200 ng/mg protein. The immunoblotting test of the crude extract of adipocytes confirmed that the immunoreactive substance in the cells was the alpha subunit of S100 protein. The treatment with 1-methyl-3-isobutylxanthine or dexamethasone alone neither elicited the S100 protein induction nor triacylglycerols accumulation in the cells. The accumulation of triacyglycerols in the cells was always preceded by the induction of S100ao protein under conditions where the differentiation to adipocytes was elicited. The induction of S100ao protein and accumulation of triacylglycerols in the cells treated with dexamethasone and 1-methyl-3-isobutylxanthine were inhibited by the addition of antimicrotubular drugs, colchicine and vinblastine, but not by cytochalasin B, an antimicrofilament drug. S100ao protein in 3T3-L1 adipocytes was released by incubation with a lipolytic hormone, adrenocorticotropic hormone or catecholamines, in a cyclic-AMP-dependent manner as observed with rat epididymal fat pads [Biochim. Biophys. Acta (1986) 889, 84-90]. These results also suggest that S100 protein may participate in the function of adipocytes.