To evaluate the in vivo antitumor activity of cleistanthin B in Ehrlich's ascites carcinoma (EAC) and Dalton's ascites lymphoma (DAL) cell lines induced malignant ascites mouse models and DAL cell line induced solid tumor mouse model. All animals were injected with 2 × 106 EAC/DAL cells i.p./s.c. to induce malignant ascites and solid tumor and treated with 5-fluorouracil (5-FU) 20 mg/kg or cleistanthin B for 10 days. Cleistanthin B was given at three doses viz. 25, 50 and 100 mg/kg. The percentage increase in life span and the overall survival in malignant ascites animals and the tumor volume in solid tumor animals were measured. The haematological parameters were assessed in all animals before and 2 weeks after the treatment. Cleistanthin B 50 mg/kg and 5-FU significantly prolonged the life span (>25%) of malignant ascites tumor bearing animals. The overall survival was significantly improved by both. Only cleistanthin B 50 mg/kg significantly reduced the elevated WBC counts in EAC tumor bearing animals. Both 5-FU and cleistanthin B 50 mg/kg reversed the malignancy induced increase in neutrophils and platelet counts and decrease in lymphocyte counts but not to the normal range. Only 5-FU significantly reduced the solid tumor volume. None of the three doses of cleistanthin B was effective against the solid tumor. Cleistanthin B has antitumor activity against EAC and DAL tumor mice but it is not as effective as 5-FU. At 50 mg/kg dose cleistanthin B exerts significant antitumor activity compared to 25 and 100 mg/kg dose. Its effect on WBC count is higher and advantageous when compared to 5-FU. But cleistanthin B in the doses used is not effective against solid tumor.