Objective: To evaluate the cytotoxic effects of Spirulina platensis extracts on acute leukemia Kasumi-1 and chronic leukemia K-562 cancer cell lines. Methods: Various concentrations of Spirulina platensis extracts (0.25–50.00 mg/mL) obtained with different solvents were used to treat cell lines for 72 h. For cytotoxic effect studies, cell viability test with trypan blue solution, MTT assay and microscopic cyto-morphological assessment were done. Results: Spirulina extract obtained with 70%ethanol showed significant cytotoxicity in K562 and Kasumi-1 cell lines. With trypan blue solution, IC50 values were found to be 4.64 mg/mL for K-562 and 3.68 mg/mL for Kusumi-1 cell lines. Spirulina aqueous extract also showed cytotoxicity with trypan blue method, at a slightly higher dose;where IC50 values were 12.68 mg/mL for K-562 and 2.13 mg/mL for Kusumi-1 cell lines. The IC50 values were found 0.40 mg/mL for K-562 and 0.31 mg/mL for Kusumi-1 cell lines for the 70%ethanol extract according to the MTT assay. Spirulina extract obtained with water also showed cytotoxicity but the dose was a little higher where IC50 values were 15.77 mg/mL for K-562 and 9.44 mg/mL for Kusumi-1 cell lines. The effect of cyto-toxicity with ethanol extract is quite comparable with that observed for cyclophospha-mide, which is a chemical used as anticancer agent. Conclusions: The cytotoxicity exhibited by Spirulina extract to cancer cell lines might be due to the presence of phytopigments (carotenoids, chlorophyll, phycocyanin) as well as polysaccharides that were reported previously as constituents of the extract. So crude extracts of Spirulina can be used as a source to develop anticancer drugs.