One way to combat or treat breast cancer, the most common cancer among women, is to decrease or prevent proliferation of cancerous cells. Many animal species of agricultural importance have been selected for various growth traits, typically through altered proliferative capacity of some cell types. Previous studies in two lines of cattle divergently selected for serum IGF-I concentration have shown that low IGF-I cattle had increased growth rates and high IGF-I cattle had decreased growth rates. Serum from either the high or low IGF-I lines of cattle were administered to MCF-7 breast cancer cell lines and doubling times were determined. The MCF-7 cells treated with serum from cattle with high serum IGF-I concentrations took 26% longer to double than MCF-7 cells treated with serum from cattle with low serum IGF-I concentrations. In conclusion, model systems employing agricultural animals may provide novel insight into mechanisms of cell proliferation.