The effects of supplementing 8% casein or 10% soy protein isolate (SPI) diets with graded levels of oligo-L-methionine (a mixture of hexa- and heptapeptides, OM) or L-methionine (Met) were studied in rats to determine the reason for the difference in nutritional quality between proteins and corresponding amino acid mixtures. As the OM concentration of the casein-based diet was increased from 0.02% to 0.6%, maximum weight gain was attained at 0.2%, and the growth-promoting activity of OM was comparable to Met at all the corresponding levels tested. Liver fat began to accumulate when supplemental Met reached a level of 0.08% of the casein diet, but OM addition did not produce a fatty liver at dietary levels of less than 0.3%. When SPI was used as the dietary protein source, the effect of supplemental OM was significantly less than that of Met. Digestibility of OM (assessed by incremental portal plasma Met concentration) was measured 30 min after feeding the casein or SPI diet supplemented with 3% OM using rats fasted for 24 h. Plasma Met concentration was greatly increased in rats fed the casein plus OM diet compared with that of rats fed the SPI + OM diet. Similarly, the 30-min portal Met concentration significantly increased in response to the casein + OM diet compared with the SPI + OM diet regardless of the prefed proteins (25% casein and 25% SPI for 2 wk).(ABSTRACT TRUNCATED AT 250 WORDS)