The ability to form gas in lactose bile-salt broth at 44.5 degrees C (the "faecal coliform" or FC test), growth in nutrient broth at 10 degrees C, indole production and pectin liquefaction were studied in 480 strains of Klebsiella isolated from human and animal infections, from various sites in the hospital environment and hospital food, and from river water and flowers. A positive FC response was correlated inversely with the ability to grow at 10 degrees C. Most strains of human and animal clinical origin were FC positive, whereas strains from water and flowers were mainly FC negative. The frequency of a positive FC response in strains from the hospital environment fell between these two extremes. The production of indole and liquefaction of pectin by klebsiellas was correlated directly with the ability to grow at 10 degrees C and a negative FC response. Nearly all of the strains could be allocated to one of four groups on the basis of these tests. The capsular serotype, bacteriocine-inhibition patterns and antibiotic sensitivities of the strains were examined. No correlation was evident between the first two properties and klebsiellas from any particular source. Strains of clinical origin were more often resistant to five or more antibiotics than were strains from the hospital environment, which in turn showed a greater frequency of antibiotic resistance than did strains from river water and flowers.