The specific activity of pyruvate carboxylase [pyruvate:carbon-dioxide ligase (ADP-forming); EC 6.4.1.1] in 3T3-L1 cells increases approximately 20-fold when these cells differentiate to an adipocyte-like form [Mackall, J. C. & Lane, M. D. (1977) Biochem. Biophys. Res. Commun. 79, 720-725]. A specific antibody to the purified rat liver enzyme quantitatively precipitated pyruvate carboxylase from 3T3-L1 crude homogenates. Use of this immunological technique permitted us to demonstrate that the increase in pyruvate carboxylase activity is due to an increase in the intracellular concentration of the enzyme. The content of pyruvate carboxylase in differentiated 3T3-L1 cells is sufficiently high (1-2% of total protein) that the increase in this large protein (subunit M(r) = 130,000) can be visualized when 3T3-L1 crude extracts are subjected to electrophoresis on sodium dodecyl sulfate/polyacrylamide gels. When 3T3-L1 cells differentiated in the presence of avidin, they contained less than 5% of the pyruvate carboxylase activity of cells that differentiated in the absence of avidin. However, the immunoprecipitable pyruvate carboxylase content of the avidin-treated cells was essentially the same as that of cells that differentiated without avidin. Full activity of the enzyme was rapidly restored in the avidin-treated cells upon the addition of excess biotin. The recovery of activity was closely correlated with the incorporation of [(14)C]biotin into immunoprecipitable pyruvate carboxylase. The rapidity with which the activity was restored and the insensitivity of the process to inhibitors of protein synthesis strongly suggest that the apoenzyme of pyruvate carboxylase accumulates during differentiation in the presence of avidin.