Streptococcal antitumor protein (SAGP) was produced by transformed E. coli JM103 carrying the SAGP gene downstream from the tac promoter. The purified recombinant SAGP had the same N-terminal amino acid sequence as that of the native SAGP isolated from Streptococcus pyogenes Su cells. Gel filtration analysis showed that the recombinant SAGP was a dimer, while the native SAGP was a tetramer. When the antitumor activity was tested against sarcoma 180 cells, the IC50 of the recombinant SAGP was 0.3 microgram/ml, about a quarter as active as the native SAGP. These results suggest that the quaternary structure of SAGP is important for the antitumor activity.