Dinitrogen fixation in white clover (Trifolium repens L.) grown in pure stand and mixture with perennial ryegrass (Lolium perenne L.) was determined in the field using N-15 isotope dilution and harvest of the shoots. The apparent transfer of clover N to perennial ryegrass was simultaneously assessed. The soil was labelled either by immobilizing N-15 in organic matter prior to establishment of the sward or by using the conventional labelling procedure in which N-15 fertilizer is added after sward establishment. Immobilization of N-15 in the soil organic matter has not previously been used in studies of N-2 fixation in grass/clover pastures. However, this approach was a successful means of labelling, since the N-15 enrichment only declined at a very slow rate during the experiment. After the second production year only 10-16% of the applied N-15 was recovered in the harvested herbage. The two labelling methods gave, nonetheless, a similar estimate of the percentage of clover N derived from N-2 fixation. In pure stand clover, 75-94% of the N was derived from N-2 fixation and in the mixture 85-97%. The dry matter yield of the clover in mixture as percentage of total dry matter yield was relatively high and increased from 59% in the first to 65% in the second production year. The average daily N-2 fixation rate in the mixture-grown clover varied from less than 0.5 kg N ha(-1) day(-1) in autumn to more than 2.6 kg N ha(-1) day(-1) in June. For clover in pure stand the average N-2 fixation rate was greater and varied between 0.5 and 3.3 kg N ha(-1) day(-1), but with the same seasonal pattern as for clover in mixture. The amount of N fixed in the mixture was 23, 187 and 177 kg N ha(-1) in the seeding, first and second production year, respectively, whereas pure stand clover fixed 28, 262 and 211 kg N ha(-1) in the three years. The apparent transfer of clover N to grass was negligible in the seeding year, but clover N deposited in the rhizosphere or released by turnover of stolons, roots and nodules, contributed 19 and 28 kg N ha(-1) to the grass in the first and second production year, respectively.