Lotus pedunculatus L., Medicago sativa L., Macroptilium atropurpureum, Glycine max, and Trifolium repens L. were grown in a N-free medium and inoculated with one of ten Rhizobium strains. Dry matter, N content, and tV5N values were determined for various plant parts. Nodules, with the exception of those from lucerne, were enriched in '5N relative to atmospheric N. Considerable variation was found in 815N values of plant herbage (-4.5 to +0.8). The extent of isotopic discrimination was dependent on both the host plant and the infecting rhizobial strain. This further complicates, but does not invalidate, the use of small variations in the natural abundance of 15N to estimate the contribution of symbiotically fixed N2 to the N in legume herbage. Some other implications of the observed differences are also discussed.