Tobramycin, a new aminoglycoside, was utilized in the treatment of 82 infectious episodes in 73 cancer patients. Forty-four episodes (54 per cent) responded to this antibiotic. Pneumonia and septicemia were the most common infections treated (62 per cent) and the response rates were 47 and 33 per cent, respectively. Tobramycin was most effective against the 11 urinary tract infections, producing a 91 per cent response rate. Gram-negative organisms were responsible for 92 per cent of the identified infections and 58 per cent of these infections responded to tobramycin.Klebsiellaspp.,E. coli, andPseudomonasspp. were the most common gram-negative bacilli causing infection. Sixty per cent ofKlebsiellainfections responded to tobramycin, compared to only 36 per cent of E. coll and 40 per cent of Pseudomonas infections. Only 24 per cent of infections occurring in patients with an initial neutrophil count of less than 1007mm3responded, whereas 69 per cent of infections occurring in patients with more than 1000 neutrophils/mm3responded to tobramycin. Nephrotoxicity was the only side effect of tobramycin and occurred during 17 per cent of the courses of therapy. However, in only 6 per cent was the drug exclusively responsible for the nephrotoxicity, Tobramycin has a spectrum of antibacterial activity similar to gentamicin and appears to be as effective in the therapy of gram-negative infections. This drug may be less nephrotoxic than gentamicin.