A technique for C2H2-reduction assay on intact plants of Alnus incana (L.) Moench was evaluated. Cloned plants were grown, in pots, on fine gravel. During assay only the pot was inserted into a Perspex incubation chamber of simple construction. The incubation volume was rather small, plants with various shoot heights could be used, and the shoot was not exposed to the C2H4 produced. Intact plants showed high and constant C2H2-reduction rates during several hours of incubation. In comparison, excised nodulated roots conventionally incubated in test tubes showed low and decreasing rates, due to removal of the photo-synthesizing shoot and injury to the root nodules when drawn from the pot. Repeated nitrogenase activity assays on the same intact individual plants did not affect growth. The technique thus proved useful in studies. where repeated nitrogenase activity measurements are important.