Cancer is the second leading cause of death worldwide.Despite their undeniable advancements, conventional therapies are still associated with serious side effects.In the recent years cancer control has exceedingly benefited from the potential that resides in alternative therapies.There is thus an increasing demand to utilize alternative concepts or approaches to the prevention of cancer.The chemopreventive effects of Chlorella sp.on the development and growth of 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary tumors were examined.Female rats were administered Chlorella at age 35 d.At age 50 d they were administered DMBA in sesame oil at 90 mg/kg body weight.They were weighed and monitored weekly for tumor development until 150 d after DMBA administration.Oral Chlorella administration of (500mg and 1000mg/kg.b.wt) produced striking reductions in the incidence, multiplicity, and volume of breast tumors relative to the control group.In the present study Chlorella showed a suppressive effect on the progression of DMBA-induced mammary tumors as well as on the levels of selected antioxidant markers.The antioxidant and pro-apoptotic properties of Chlorella may contribute to its protective effects.