SUMMARY: A method is described for the preparation of highly competent (transformable) bacteria of Haemophilus influenzae. After three successive treatments, i.e. aerobic growth, anaerobic incubation and incubation in saline containing a few per cent broth, transformation frequencies close to 5% have been obtained. Competent bacteria absorb DNA very fast. Untransformable ones lack this ability. Absorption is inhibited by dinitrophenol or arsenate; it is not inhibited by chloramphenicol. This suggests the participation of an enzyme system. Development of competency is blocked by dinitrophenol, arsenate or chloramphenicol. It is strongly dependent upon temperature. It is not influenced by the absorption of one or two DNA molecules per bacterium.