An experiment is described which investigated the combined effects of fertilizer nitrogen and a medium-large-leaved variety of white clover on the production from a mixed sward. Over a period of 3 years six rates of nitrogen fertilizer ranging from 0 to 750 kg/ha were applied annually on three different sward types cut five times per year. The swards consisted of S. 23 perennial ryegrass alone, S. 23 ryegrass plus Blanca white clover, and Blanca white clover alone. Averaged over the 3 years the nitrogen rate required on the pure-grass sward to give the same yield of dry matter as the grass plus Blanca clover sward with no fertilizer nitrogen applied was 265 kg/ha; the corresponding application rate to achieve equal crude-protein yield was 322 kg/ha. Blanca had an additive effect on the yield from the mixed sward at nitrogen rates up to at least 300 kg/ha. The results from this experiment are compared with those from experiments in which medium-small-leaved varieties of white clover were used. The role of white clover in providing savings in nitrogen fertilizer input on grassland is discussed. Estimates from the results indicate that the nitrogen rates required to produce an annual herbage dry-matter yield of 12 t/ha were 340 kg/ha on the pure ryegrass swards, and 140 kg/ha on the ryegrass plus Blanca sward. The nitrogen fertilizer saving due to the inclusion of Blanca white clover in the sward was, therefore, 43%. A similar estimate from the results of an earlier experiment with the medium-smallleaved variety Aberystwyth S. 100 suggests a nitrogen fertilizer saving of 21%.