Human lactate dehydrogenase (LDH) was universally detected and quantified from sera of nude mice bearing human tumors. The patterns of LDH isozyme in sera reflected exactly those in each tumor extract, and remained unchanged over several generations. In 16 human cancers (7 gastric, 4 colorectal, 2 pancreatic and 3 breast) examined, human LDH levels in serum showed proportional increase as the transplanted tumors grew. Assessment of experimental chemotherapy by measuring the LDH levels of a treated group versus a control group was compared with that obtained by calculating the tumor weight during (14 tests) or after (61 tests) treatments. The percentage inhibition assessed by LDH level was strongly correlated to inhibition rate assessed by tumor weight. Moreover, LDH expressed more rapidly the inhibitory effect of an administered agent than tumor weight. If the drug was effective, serum LDH level in the treated group exhibited a temporary sharp rise within 2 days, thereafter decreasing to a lower level than that of the control. These results imply usefulness in further clinical application.