Casein is a major protein in cow's milk that occurs in several variant forms, two of which are beta-casein A1 and beta-casein A2. The levels of these two proteins vary considerably in milk dependent on the breed of cow, and epidemiology studies suggest that there is a relationship between their consumption and the degree of atherosclerosis. In the present study, the direct effect of consumption of beta-casein A1 vs beta-casein A2 on atherosclerosis development was examined in a rabbit model. Sixty rabbits had their right carotid artery balloon de-endothelialised at t=0, divided randomly into 10 groups (n=6 per group), then for 6 weeks fed a diet containing 0, 5, 10 or 20% casein isolate, either beta-casein variant A1 or A2, made up to 20% milk protein with whey. Some groups had their diets supplemented with 0.5% cholesterol. Blood samples were collected at t=0, 3 and 6 weeks and rabbits were sacrificed at t=6 weeks. In the absence of dietary cholesterol, beta-casein A1 produced significantly higher (P<0.05) serum cholesterol, LDL, HDL and triglyceride levels than whey diet alone, which in turn produced higher levels than beta-casein A2. Rabbits fed beta-casein A1 had a higher percent surface area of aorta covered by fatty streaks than those fed beta-casein A2 (5.2+/-0.81 vs 1.1+/-0.39, P<0.05) and the thickness of the fatty streak lesions in the aortic arch was significantly higher (0.04+/-0.010 vs 0.00, P<0.05). Similarly, the intima to media ratio (I:M) of the balloon injured carotid arteries in A1 fed animals (0.77+/-0.07) was higher than in those that consumed A2 (0.57+/-0.04) or whey (0.58+/-0.04), but this did not reach significance. In the presence of 0.5% dietary cholesterol, the thickness of the aortic arch lesions was higher (P<0.05) in 5, 10 and 20% casein A1 fed animals compared with their A2 counterparts, while other parameters were not significantly different. It is concluded that beta-casein A1 is atherogenic compared with beta-casein A2.