RHIZOBIA can infect legume roots and fix dinitrogen in the root nodules. Repeated attempts have failed to demonstrate fixation by rhizobia on defined medium or in plant extracts. Rhizobium japonicum grown anaerobically with nitrate produces a haemoprotein pattern similar to that of N2-fixing bacteroids1 and synthesises the Mo–Fe protein of nitrogenase2, but it has seemed that infection of legume cells was a prerequisite for the expression of nitrogenase. It has been shown recently3,4, however, that cowpea Rhizobium 32H1 could fix dinitrogen when grown close to, but not in contact with, callus cultures of legumes or non-legumes. This supported other evidence5,6 that it was the rhizobia which contained the genetic information for nitrogenase, and showed that its expression was promoted by diffusable factors from plant cells.