A new sulfonamide derivative, the 2-p-aminobenzene-sulfonamido-3-methoxy-pyrazine, is very active against Dipiococcuspneumoniae and Staphylococcus aureus infections in the mouse. The therapeutic activity of this new compound is higher than that of other analogous derivatives (3-sulfanilamido-6-methoxypyridazine (SMPd), 4-sulfanilamido-2, 6-dimethoxy-pyrimidine (SDMP) and 3-sulfanilamido-2-phenyl-pyrazole (SF)). It is useful in the treatment and prophylaxis of bacterial infections where it can be given in low doses since it remains for a long time and at high concentrations in the circulating blood.