A combination of physiological and structural measurements made on nodulated cowpea and soybean plants cultured with roots in different pO(2) permitted the expression of data in various ways. Values of leghemoglobin concentration and nitrogenase activity from the two legumes were expressed conventionally either on a per plant or per gram nodule fresh weight basis, and where microscopy was done, on the basis of nitrogenase-containing, N(2)-fixing units (i.e. per bacteroid, per infected cell, or per gram infected tissue). In both legumes, acetylene reduction, N fixed and ureide content expressed on the basis of whole plants or per nitrogenase-containing units were very significantly correlated with values of leghaemoglobin concentrations expressed in a similar manner. The use of mathematical correlations in this study involving leghaemoglobin concentrations and various indices of N(2) fixation indicated a strong functional relationship between the two proteins in symbiotic legumes. These findings confirm previous suggestions that leghaemoglobin and the nitrogenase complex are two proteins closely associated with N(2)-fixing efficiency in legume root nodules.