Serine:glyoxylate aminotransferase (EC 2.6.1.45) from green parts of 7-day-old rye seedlings was purified 600-fold. Specific activity of the purified enzyme against L-serine and glyoxylate as substrates was 53.2 mumol/mg protein per minute at 30 degrees C. The enzyme activity with L-alanine or L-asparagine and glyoxylate, or with L-asparagine and hydroxypyruvate was 20% that with L-serine and glyoxylate as the amino group acceptor, whereas with L-alanine or glycine and hydroxypyruvate it was 10% of that value. The reaction rate with pyruvate and L-asparagine, glycine or L-serine was very low. The enzyme was stabilized by the presence of sucrose, pyridoxal phosphate and 2-mercaptoethanol. Molecular sieving of the native enzyme on Sephacryl S-300 gel gave Mr values of 91,200 and 85,000, whereas the molecular weight estimated by SDS-polyacrylamide gel electrophoresis was 43,000, indicating the dimeric structure of the enzyme.