The changes in rabbit serum proteins after partial hepatectomy were examined by means of two-dimensional electrophoresis utilizing isoelectric focusing in a 4% polyacrylamide gel in the first dimension and a 4-30% pore gradient polyacrylamide gel in the second dimension. A rapid increase in seven proteins was observed after partial hepatectomy and a rapid decrease in two proteins. Major serum proteins, including albumin, immunoglobulin G, immunoglobulin M and alpha 2-macroglobulin, did not change. The time course of the changes was examined using a densitometer; the maxima of the changes were observed on day 3 after partial hepatectomy.