The human tumor clonogenic assay (HTCA) is a double-layer agar technique, which provides an in vitro prediction of the response of an individual patient's tumor to an antitumor agent. This paper briefly provides an outline of HTCA and its potential use in chemotherapy on patients with lung cancer. In our experience with culturing 123 carcinomas of the lung, 105 specimens (85%) could be subject to more than 5 chemosensitivity tests each by modifying the preparation method of single cell suspension in this system. Growth rate was improved in all types of primary human lung cancer with reasonable consistency. Further, metastatic tumors were capable of being successfully grown in a high percentage of cases, which was comparable to the results obtained for other kinds of tumors. There was no correlation of growth or cloning efficiency with histology, source, or previous chemotherapy. Using 50% or more inhibition on to colony formation as the criterion for chemosensitivity, response rates to vindesine or mitomycin C were 19% or 16%, respectively. The in vitro response rates of these or almost all other antitumor drugs seemed to be comparable to the clinical responses reported by various investigators. Correlation between in vitro chemosensitivity in HTCA and clinical response has been evaluated by various investigators, and the pooled data have demonstrated a good association between in vitro drug sensitivity and clinical response or lack of response. In lung cancer, HTCA had a 57% true positive rate and an 85% true negative rate for the prediction of drug sensitivity and resistance, respectively, of cancer patients to specific chemotherapeutic drugs. Although the system still has to undergo modification to resolve a number of theoretical and practical problems, it has potential uses in lung cancer chemotherapy.