Our objective was to evaluate the cancer suppression activity of extracts from a commercial variety of yellow-fleshed peach 'Rich Lady' (RL) and a red-fleshed plum 'Black Splendor' (BS) and identify the phenolic fractions that may possess potential as chemopreventive and/or chemotherapeutic natural compounds. The peach RL extract effectively inhibited the proliferation of the estrogen-independent MDA-MB-435 breast cancer cell line. The concentration to inhibit 50% of cell proliferation (IC(50)) was approximately 42 mg/L for this cell line compared to an IC(50) of approximately 130 and approximately 515 mg/L for the noncancerous breast cell line MCF-10A and the estrogen-dependent breast cancer cell line MCF-7, respectively. Similarly, BS extracts showed greater effects on MDA-MB-435 cells as compared to the other breast cancer or the normal breast cell lines. In general, BS extracts were less effective than RL extracts. Within all RL and BS fractions, fraction 3 (F3, flavonoids) and fraction 4 (F4, procyanidins) were more potent than fraction 1 (F1, phenolic acids) and fraction 2 (F2, anthocyanins) against the three cell lines. The order of potency of RL fractions against MDA-MB-435 was F(3) approximately F(4) > F(1) > F(2). The antiproliferative activity of pure compounds identified in F(3) and F(1) confirmed that quercetin 3beta-glucoside is the bioactive compound in F(3), with the same level of toxicity on the estrogen-independent MDA-MB-435 breast cancer and breast epithelial MCF-10A cells (IC(50) = 1.9 +/- 0.2 and 1.8 +/- 0.3, respectively). However, we confirmed that phenolic acids present in F(1): chlorogenic and neo-chlorogenic acids have potential as chemopreventive dietary compounds because of the relatively high growth inhibition exerted on the estrogen-independent MDA-MB-435 breast cancer cell line and low toxicity exerted in the normal MCF-10A cells.