Biogenesis of tetrahydrofolate cofactors essential for bacterial growth and survival is blocked by sulfamethoxazole-trimethoprim. An intravenous form of the antimicrobial combination has recently been approved for the treatment of acute, symptomatic, bacterial pyelonephritis, recurrent urinary tract infections, shigellosis, and Pneumocystis carinii pneumonia. Intravenous sulfamethoxazole-trimethoprim has emerged as an invaluable agent for the management of selected infections, including bacterial meningitis and Salmonella bacteremia, where limited therapeutic alternatives exist. In addition, co-administration of intravenous sulfamethoxazole-trimethoprim with a carboxypenicillin provides an empiric treatment for the infected granulocytopenic patient that compares favorably with standard combinations. Adverse events unique to the intravenous form of the drug consist of phlebitis and fluid imbalances. Fluid overload results from the relatively large volumes of 5% dextrose solution required as diluent.