Background: Saccharomyces cerevisiae, a common probiotic, can induce in vitro apoptosis in human cancer cells, which could explain its antitumor activity. Objectives: The present study was conducted to investigate the in vitro effects of a cytoplasmic extract from S. cerevisiae on the proliferation and viability of a K562 (chronic myeloid leukemia) cell line. Methods: S. cerevisiae was cultured and then disrupted by sonication. After centrifugation, the harvested supernatant was considered to be a cytoplasmic extract. The protein concentration was determined by the Biuret method and the extract was diluted to