1. Earlier studies have shown that exposure of fat-cells to insulin results in the rapid increased phosphorylation of an acid-soluble 22 kDa protein and that increases in phosphorylation were also evident in cells exposed to adrenaline [Belsham & Denton (1980) Biochem. Soc. Trans. 8, 382-383; Belsham, Brownsey, Hughes & Denton (1980) Diabetologia 18, 307-312]. 2. The effects of adrenaline are shown to be brought about through beta-adrenergic receptors and to be mimicked by other agents which increase cell cyclic AMP concentrations. The maximum extent of phosphorylation is about 60% of that observed with insulin. Increased phosphorylation is also observed in fat-cells exposed to vasopressin, oxytocin and phorbol esters, but not to alpha-adrenergic agonists. 3. No changes in the phosphorylation of the protein are evident in epididymal fat-pads from fat-fed, starved or starved/refed animals, despite the large changes in protein composition of fat-cells which accompany these nutritional alterations. This suggests that the protein is not closely involved in lipogenesis or associated metabolic pathways, but rather that it may play a more general regulatory role. 4. The 22 kDa protein migrates as a doublet on SDS/PAGE even after purification to apparent homogeneity by sequential use of Mono Q chromatography, SDS/PAGE and h.p.l.c. The amino acid compositions of the two components are very similar and share features in common with a number of proteins, including inhibitor-1, inhibitor-2, dopamine- and cyclic-AMP-regulated phosphoprotein (DARPP-32), and G-substrate, which may be involved in the regulation of protein phosphatase activity. 5. Phosphopeptide mapping and phosphoamino acid analysis reveals that insulin increases the phosphorylation of two distinct peptides within the protein (in one peptide insulin increases the amount of phosphothreonine, whereas in the other the hormone increases the amounts of phosphothreonine and phosphoserine). Both components of the doublet exhibit similar changes in phosphorylation, and hence the differences in migration are not the result of differences in phosphorylation, as suggested previously [Blackshear, Nemenoff & Avruch (1983) Biochem. J. 214, 11-19]. The pattern of phosphorylation observed with the beta-adrenergic agonist isoprenaline was similar to that observed with insulin. 6. The possible role and regulation of the 22 kDa protein are discussed.