The cytologic examinations and cloning efficiencies of 47 human tumor specimens (19 ovarian carcinomas, 6 sarcomas, 5 lung carcinomas, and 17 miscellaneous tumors) were compared to evaluate the predictability of clonogenecity by cytologic diagnosis. Cytologic examination preceding clonogenic assay identified the 50% to 60% of all specimens that failed to yield in vitro chemotherapy sensitivity by this assay, with a false-negative rate of 10%. The frequency of cytologically identifiable tumor cells in the plating suspension was independent of the histologic type of tumor (ovarian carcinoma or others) and the nature of the specimen (solid tumor or malignant effusion). Nearly 66% of all specimens were cytologically negative. The in vitro sensitivity can be assessed in approximately 75% of cytologically positive specimens. Cytologic evaluation preceding soft-agar clonogenic assay of human tumors can reduce cost and effort by predicting colony-producing specimens.