Our objective was to determine whether dietary plant proteins such as soya-protein isolate (SPI) and rice-protein isolate (RPI) compared with animal proteins, such as casein, could afford beneficial effects on atherosclerosis development in apolipoprotein E-deficient mice. In experiment 1, male and female mice were fed on a purified diet containing either casein, SPI or RPI for 9 weeks. The en face lesion area in the aorta (P < 0·05) and the lesion size in the aortic root (P < 0·05) in mice fed the casein-based diet were greater than those in the SPI or RPI groups. The plant protein groups had an increased concentration of serum l-arginine (P < 0·05) and NO metabolites (NO2 plus NO3) (P < 0·05) than did the casein group. The inhibitory effect of the plant proteins on the lesion formations was unrelated to gender and total serum cholesterol. In experiment 2, the l-arginine and l-methionine contents were the same in the l-arginine-supplemented casein-based and SPI-based diets, and between the l-methionine-supplemented SPI-based and the casein-based diets. Male mice were fed on the diets for 15 weeks. There were no significant differences in the en face lesion area and the lesion size between the casein group and the l-arginine-supplemented group, although the serum l-arginine (P < 0·05) and NO2 plus NO3 (P < 0·05) concentrations in the supplemented group were higher than those in the casein group. There were no significant effects of l-methionine supplementation on the lesion formations. In experiment 3, male mice were given the casein-based diet or the l-arginine-supplemented casein-based diet together with water or water containing an NO synthesis inhibitor for 9 weeks. When given the casein-based diet, the inhibitor drinking, compared with water drinking, resulted in a reduction of the serum NO2 plus NO3 concentration (P < 0·01) and an increase in the en face lesion area (P < 0·05) and the lesion size (P < 0·01). When given the l-arginine-supplemented diet, the inhibitor drinking, compared with water drinking, resulted in no increase in the lesion area and size. These results demonstrate anti-atherogenic potentials of SPI- as well as RPI-derived proteins, but their l-arginine and l-methionine contents were not sufficient enough to explain the underlying mechanism(s).