Practical criteria were developed in this paper for the purpose of evaluating chemosensitivity of fresh human breast cancer by the MTT assay. The survival rates at maximum inhibition (Imax %) and the concentrations of drugs which caused fifty percent reduction in absorbance compared to baseline values (IC50) of 175 samples of 10 anti-tumor drugs were evaluated by logistic analyses of the dose-response curves. Distributions of Imax% appeared as normal curves, while those of the IC50 significantly deviated from normal distribution (p < 0.0001). We assessed the in vitro chemosensitivity by comparing the Imax % of each drug on individual samples with the mean Imax % + SD which was obtained from the Imax% of 175 samples. If the individual Imax % > mean Imax % + SD. we thought the tumor sample was resistant to this drug. If the Imax % < or = mean Imax % + SD, we would compare its IC50 with Q50 which was used as a cutoff point for in vitro chemosensitivity of anti-tumor drugs. The in vitro chemosensitivity could be graded as sensitive (Q1-Q25), intermediate (Q26-Q75), and resistant (Q76-Q100) by means of percentile method. If the individual IC50 > or = Q76, the tumor sample would be defined as resistant. If the individual IC50 < or = Q25, it would be defined as sensitive. In the range of Q26-Q75, we used Q50 as a cutoff point between relative sensitivity and relative resistance. Preliminary results showed that the in vitro chemosensitivity to different anti-tumor drugs determined by these criteria were consistent with the clinical response in 83 advanced breast cancer patients.