The possible role of Klebsiella species on intestinal infection pathology was investigated. From a total of 230 strains isolated during 1979 and 1980 from different hospitals in Rio de Janeiro City 91.0% corresponded to Klebsiella pneumoniae an d9.0% were Klebsiella ozaenae. In 10.43% of the cases the Klebsiella strains were found in close association with a definitive pathogeny, belonging the other genera. Besides, parasitological stool examination performed in 43 patients revealed helminthic and/or protozoan agents in 4.0% of the sample. The significance of other meaningful mechanisms, excluded the capsule, to the pathogenic role Klebsiella was investigatedl. One hundred eighty seven strains were tested for the heat stable enterotoxin (ST) but in only two cases a borderline result was obtained, with no significant results in the remaining strains. The heat labile enterotoxin (LT) was investigated in 110 strains, with no positive results. Finally, the colonization factor CFA/1 was not identified in 27 strains tested.