Background: Plants are a proven source of anti-tumor compounds and it is reasonable to assume that many such substances remain to be discovered. In Iranian folk medicine, ethanolic extract of the aerial parts of Daphne mucronata Royle is used against various skin disorders.Objective: In order to evaluate anti-neoplastic plants, we initiated a cytotoxic study using Daphne mucronata that is indigenous to Iran.Methods: The cytotoxic activity of two hydroalcoholic and chloroformic extracts of Daphne mucronata Royle were examined on seven different cell lines by MTT assay. Cell lines used in this study included SK-Br-3 and MDA-MB-435 (breast cancer), Hela (cervical epitheloid carcinoma), K562 (myelogenous leukemia), U937 (monoblastic leukemia), Ag.8 (mouse myeloma) and Vero (primary monkey kidney).Results: The highest cytotoxic activity of the hydroalcoholic extract of D. mucronata was found on breast cancer cell lines. 50 mg/ml of the extract inhibited proliferation of 24-hour cultures of MDA-MB-435 and SK-Br-3 cell lines (73% and 34% inhibition, respectively). The extract showed anti-leukemic activity particularly against the U937 cell line. A 50% inhibition of cell proliferation due to 100 mg/ml of the extract in 24-hour culture of U937 and 48-hour culture of Ag.8 cell lines was observed. Despite the result of MTT assay showing a reduction of Hela cell line viability after 24-hour exposure to 10-50 mg/ml of the extract, a significant stimulatory activity at concentrations more than 400 mg/ml was noted. No significant cytotoxic effect was detected in relation to the Vero cell line. The chloroformic extract showed weak cytotoxic effect on MDA-MB-435, SK-Br-3 and U937 cells but had no significant effect on the other cell lines.Conclusion: Hydroalcoholic extract of D. mucronata showed anti-tumour activity particularly against breast and leukemia cell lines.