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Oligo-L-methionine ethylester (OMOEt) prepared by the papain-catalyzed oligomerization of L-methionine ethylester (MetOEt) is a mixture of pentamer to dodecamer and has nearly the same supplementary effect as free methionine (Met) for the growth of rats when added to a low casein diet, but its supplementary effect to a low-soy protein isolate (SPI) diet is not consistent and depends on the degree of polymerization. Rats were fed for 2 wk with an 8% casein or 10% SPI diet supplemented with 0.3% L-Met, each chemically synthesized MetnOEt with a polymerization degree (n) of 6, 7, 8, or 9, or with OMOEt prepared by papain-catalyzed polymerization of MetOEt. Met6OEt, Met7OEt, and Met8OEt had nearly the same supplementary effect on the growth of rats, as did free Met, both with the 8% casein and 10% SPI diets. The supplementary effect of Met9OEt was not significantly lower than that of Met when added to the 8% casein diet, but was significantly lower when added to the 10% SPI diet. The digestibility of Met9OEt supplemented to the 8% casein and 10% SPI diets was 50.5% and 35.6%, respectively. It appears likely that there is a gap in the bioavailability of oligomethionine between the octamer and nonamer when added to a low-protein diet, probably due to the rigidity of the structure increasing with the polymerization degree by alpha-helix formation. Although the differences in absorption rate of Met from OMOEt for a short time after feeding has been related to the different effects of supplemented OMOEt, the absorption rate of OMOEt for 30 min after feeding was not considered to be the main cause of the differential effects of OMOEt in this experiment.