We have developed an in vitro drug screening test by scintillation assay with which we can assess the chemosensitivity of solid human tumors during the 5 days following tumor resection. We are able to evaluate 62% (271/439) of the human tumors' chemosensitivity results. Seventy-seven of these cases were examined for in vitro-in vivo correlations. In 14 of 33 in vitro sensitivity cases (42%) we found clinical responses. And, in 41 of 44 in vitro resistance cases (95%) we found progressive disease. These in vitro-in vivo correlations with the scintillation assay were similar to those in other reports with different types of chemosensitivity assay. The true-positive accuracy of these tests averaged 50%, while the true-negative accuracy averaged 90%. We think that tumor cell heterogeneity is one of the main reasons for low true positive accuracy in the drug screening assays. In some studies we implanted human tumors into nude mice. The resulting xenografts demonstrated that fresh human tumors are composed of heterogeneous subpopulations of cells with varying chemosensitivities and varying DNA indexes. This heterogeneic nature of human tumor cells means that a single biopsy specimen may not comprehensively define the chemosensitivity of a patient disease.