Effects of intact and processed bovine milk proteins on development of chemically induced mammary tumors in female rats were compared. AIN-93G diets were made with 20% casein (CAS), casein hydrolysate (CASH), intact whey protein (IWP), or whey protein hydrolysate (WPH). Pregnant Sprague-Dawley rats were fed the diets starting at Gestational Day 4. Offspring were fed the same diet. At 50 days, female offspring (44-49/group) were gavaged with sesame oil containing 80 mg/kg of the mammary carcinogen dimethylbenzanthracene (DMBA) and euthanized 62 days posttreatment. Rats fed WPH had an adenocarcinoma incidence of 17% compared to the rats fed CAS, CASH, and IWP diets (34%, 33%, and 36% respectively) (P < 0.001). Median palpable tumor latency for rats fed WPH was greater (61 days, P < 0.001) compared to CAS (44 days), CASH (42 days) and IWP (45 days). Tumor multiplicity was also lower (1.5 vs. 3.0, P < 0.05) in rats fed WPH than in CAS and CASH fed groups. Results demonstrate that hydrolytic processing of whey protein is required for this diet to be effective in reducing DMBA-induced mammary tumors. The bioactive compounds produced during whey protein processing and mechanisms underlying the anticancer effects of WPH are yet to be identified.