A comparison was made of growth and drug-response of five human tumor cell lines (HT-29, colon carcinoma; TWI, melanoma; A-549, lung carcinoma; Panc-1, pancreatic carcinoma; and EJ, bladder carcinoma) in serum-free media (SFM) and in serum-supplemented media (SSM) using the human tumor-clonogenic assay (HTCA) system. HT-29 cells, which had the highest plating efficiency in both SFM and SSM, were used to obtain dose-response curves for four drugs (adriamycin, 5-fluorouracil, cisplatin, and BCNU) in the HTCA. Three of the drugs (adriamycin, 5-fluorouracil, and cisplatin) produced identical drug-response curves in both SFM and SSM. These results suggest that, for some chemotherapeutic agents, results comparable to those obtained with SSM in the HTCA can be achieved using SFM. Step-by-step addition of growth factors and hormones to SFM may be a useful technique to improve some of the technical and logistic problems associated with the HTCA.