Very little research has been attempted to determine isoleucine (Ile) requirements of late-finishing pigs. Variability in these rapidly growing animals makes interpretation of results a very difficult task. Attempts to minimize variability by using short-term feeding assays have resulted in trials that utilize minimum plasma urea-nitrogen as an indicator of the requirement (Liu et al., 2000a). Other research by Liu et al. (1999) has shown that reducing protein level and adding amino acids (AA) does not affect growth performance of early- or late- finishing pigs. Their results have also suggested that Ile is a limiting AA for late-finishing barrows fed AA-fortified, low-protein corn diets (Liu et al., 2000b). When diets (7.1% CP) comprised of corn (supplemented with synthetic L-lysine, L-threonine, L-tryptophan and DL-methionine) were fed to late-finishing pigs, growth responses to supplemental Ile were observed. This suggests that Ile may become a limiting AA for late-finishing pigs fed low-protein diets. Therefore, it is important to determine the Ile requirement of late-finishing pigs.