Freshly isolated clinical strains of Klebsiella were tested for the ability to fix dinitrogen by the acetylene reduction assay. Ability to detect this trait was markedly affected by cultural conditions. When the test was run at 37 degrees C in the presence of yeast extract (50 mg/liter), only 1.6% of the organisms were diazotrophs, whereas this temperature without yeast extract yielded 12.9% positive cultures. The optimum condition found was 28 degrees C without yeast extract (21.9% positive); therefore, search for diazotrophy in clinical strains should not be conducted at the usual incubation temperature. There was a high incidence of indole-positive strains among diazotrophs. No such correlation was noted with any other biochemical trait or antibiotic susceptibility tested. The significance of this correlation is not apparent.