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Nodule‐specific root proteins – so called nodulins – were identified in root nodules of pea plants by an immunological assay. Nodulin patterns were examined at different stages of nodule development. About 30 nodulins were detectable during development. Some were preferentially synthesized before nitrogen fixation started, whereas the majority were synthesized concomitantly with leghaemoglobin. Some of the nodulins were located within the peribacteroid membrane. Ineffective Rhizobiumstrains (a natural nod+fix‐and a pop ‐fix‐) appeared to be useful in studying the expression of nodulin genes. Synthesis of some nodulins was repressed in ineffective root nodules, indicating that nodulins are essential for the establishment of nitrogen fixation. In both types of ineffective root nodules, leghaemoglobin synthesis was not completely repressed. Low amounts of leghaemoglobin were always detected in young ineffective root nodules whereas in old nodules no leghaemoglobin was present.