The pseudomonad EO-1 (Eugonic Oxidizer Group 1) or Pseudomonas cepacia, multivorans, or kingii was isolated from 41 patients over a two-year period at the University of Minnesota Hospitals. All patients had significant underlying medical problems. Thirty-seven of the 41 patients had prior instrumentation or a manipulative procedure that directly involved the body site from which the organism was isolated. The mean duration of hospital stay at the time of initial isolation was 19 days (median, 14 days and range, one to 127 days). Sixty-nine percent of patients were receiving one or more antibiotics at the time P. cepacia was first isolated. These strains were consistently susceptible only to the sulfonamides, chloramphenicol, and naladixic acid and were routinely resistant to gentamicin and the polymyxins.