Intact rat epididymal fat-cells were incubated with 32Pi and the intracellular proteins separated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. One of the phosphorylated proteins has the same RF value as [14C]biotin-labelled acetyl-CoA carboxylase purified from fat-cells and is specifically precipitated after incubation with antiserum raised against acetyl-CoA carboxylase. No significant changes in the extent of phosphorylation of acetyl-CoA carboxylase were detected after exposure of the cells to insulin.