Four patients in a leukaemic ward were infected with multi-resistant Pseudomonas aeruginosa. Similar organisms were found in the environment and it appeared that lapses in aseptic routine contributed to the outbreak. Serological, bacteriophage and pyocin-typing revealed that a fifth patient was infected with a distinct strain, but agarose gel electrophoresis indicated that all patient and environmental strains carried the same plasmid. The plasmid had a molecular weight of 47 (s.d. +/- 2) X 10(6) dal and was transfer deficient. It conferred resistance to carbenicillin, gentamicin, kanamycin, streptomycin, sulphonamide, tetracycline and tobramycin and determined an aminoglycoside adenylyltransferase active against amikacin in-vitro and not in-vivo. Spread of this non-transferable plasmid to a different Ps. aeruginosa strain and dissemination of multi-resistant organisms led to serious infections.