The epidemiological and biochemical characteristics of Pseudomonas aeruginosa strains causing septicemia in a Spanish hospital over a ten-year period (1981-1990) were analyzed. A total of 207 episodes, corresponding to 0.7 episodes per 1,000 inpatients and 3.2% of the total number of episodes of septicemia, were registered. Males were more often affected than females (rate 3.2:1). The respiratory (24.6%) and urinary (21.2%) tracts were the main portals of entry, while haematologic and solid tumours (15.4%) were the most frequent underlying diseases. More than 86% of the strains were susceptible to ceftazidime, mezlocillin, piperacillin and amikacin. Seventy strains were subjected to typing and analysis of virulence factors. Serotypes O:6, O:11 and O:2 could be considered endemic (each present in more than 11.4% of strains). Pyocin typing, antibiotyping and resistotyping were preferred as secondary typing methods to phage typing and plasmid profile analysis. The combination of methods revealed a large diversity of strains although some clusters predominated. More than 80% of the strains produced several exoenzymes, possessed pyoverdin and showed haemolytic activity, and all except one showed serum resistance. All strains were susceptible to silver and more than 80% to mercury and boron, but all were resistant to iodine.