SummaryA soft agar clonogenic assay for human tumor stem cells was applied forin vitroassessment of the chemotherapeutic value of both standard and experimental antineoplastic agents. Fifty-six samples of unselected human malignancies were tested and a successful oncogram, defined as a prediction of sensitivity or resistance to at least two cytostatic agents was obtained in 23 % of the cases. Partially overlapping causes of failure were 1) no colony growth 50%; 2) unevaluability of colony growth due to cell clumps 9 %; 3) not enough cells 20 %; 4) infection 7 %; and 5) enough colonies 5 %. The predictive value of the human tumor stem cell clonogenic assay (HTSCA) appeared to be excellent for resistance and fair for sensitivity. In addition, the HTSCA has been shown to be able to detect growth promotionin vitroinduced by sex hormones. To circumvent causes of failure 2) and 3), an alternative assaying procedure replacing visual counting by (3H) thymidine incorporation was developed. It was shown to be readily applicable to human tumor material.