Mushrooms are part of the sexual life cycle of particular fungi with specific metabolic pathways, and therefore may contain a largely unexploited source of powerful new pharmaceutical products with potential antitumor properties [1,2]. Furthermore, they may have potential as functional foods. Suillus collinitus is an edible mushroom found in European pine forests. The aim of this work was to study the cytotoxic potential of extracts from this mushroom in various cancer cell lines. Different extracts (methanolic, ethanolic and aqueous) were prepared and extractinduced cell growth inhibition was assessed with the sulforhodamine B assay in four human tumour cell lines (lung, breast, colon and gastric cancer). The methanolic extract was further characterized in its phenolic composition by HPLC-DAD. The effects of the extract on cell cycle profile and apoptosis were evaluated by flow cytometry and the effect on the expression levels of proteins related to cell cycle and apoptosis was further investigated by Western blotting. Regarding cell growth inhibition, the methanolic extract was the most potent one, particularly in MCF-7 cells (GI50 25.2±0.2lg/ml). Moreover, the GI50 concentration ninduced a G1 cell cycle arrest, with a concomitant decrease in the percentage of cells in the S phase. Furthermore, it caused an increase in the percentage of apoptotic cells, from 6.0±0.2% in untreated cells, to 15.3±2.0% in treated cells. In addition, 48h treatment with the GI50 concentration caused a strong increase in the levels of p53, p21, cleaved caspase-3 and cleaved PARP, together with a decrease in Bcl-2. The main components identified in the methanolic extract were: protocatechuic acid (5.2±0.2mg/kg dw), p-hydroxybenzoic acid (14.1±1.2mg/kg) and cinnamic acid (1.3±0.2mg/kg). Results indicate that Suillus colinitus is a promising source of bioactive compounds. Particularly, its methanolic extract appears to have a p53-mediated effect on the normal cell cycle distribution and apoptosis induction in human breast tumor cells.