Abstract: AIM: To study the effects of different number of tumor cells on xenograft tumor growth and efficacy of anti-tumor agents inmice.METHODS: Different numbers of tumor cells were injected subcutaneously at the right armpit of the mice.The tumor volume and weight were determined.The efficacies of four anti-tumor agents were evaluated.RESULTS: The more the number of tumor cells implanted, the faster the xenograft tumor growth.The earlier the tumor emerged, the lower the inhibitory rate of antitumor agents showed.CONCLUSION: The number of tumor cells implanted obviously affects the growth of xenograft tumor in mice and lead to misevaluate in efficacy of anti-tumor agents.So the attention should be paid when the efficacies of different antitumor agents are evaluated.

Key words: tumor load, mouse, tumor growth, antitumor agents, evaluation