Since November 1969 the Baltimore Cancer Research Center has used the combination of carbenicillin (C) and gentamicin (G), with or without cephalothin, as therapy for granulocytopenic patients with severe Gram-negative bacillary infection, especially Pseudomonas aeruginosa. Since 1971 an increasing percentage of isolates have been resistant to carbenicillin or gentamicin, or both, by disc-susceptibility testing. Measurements of minimum inhibitory concentrations showed that, during 11 months, 14 patients, 8 with acute leukemia, had developed resistant strains. From these 14 patients, 40 distinct serotypes or resistance patterns of P. aeruginosa could be delineated: 15 were C- and G-susceptible; 12, C-resistant and G-susceptible; 5, G-resistant and C-susceptible; and 8, C- and G-resistant. The appearance of resistant organisms correlated significantly with the administration of carbenicillin or gentamicin, or both. The induction of gentamicin resistance was particularly associated with oral G intake. Carbenicillin resistant variants and strains susceptible to carbenicillin and gentamicin were virulent, but those with gentamicin resistance seemed to be less invasive.