Tissue homogenates of 10 kinds of human cancer tissues were incubated in medium containing either one of C¹⁴-1, C¹⁴-2, or C¹⁴-3-lactate as a substrate in order to observe the oxidative pathway of lactate in cancer tissues. Lactate concentration in incubation medium was maintained at 50 mg%. At the end of incubation period, gas samples and incubation media were analyzed for total CO₂ production rates, radioactivities of respiratory CO₂, lactate uptake rates and pyruvate appearance rates. The following results were obtained. 1. Lactate uptake rates in all of cancer tissues examined were less than 2.5 μM/hr/gm and much lower than those in normal tissues. 2. In the 10 kind of human cancer tissues, total CO₂ production rates were less than 10 μM/hr/gm, in all cases. These lower values impressed that oxidative metabolism in tumor tissues generally inhibited as compared with that in normal tissue. On the other hand, fractions of CO₂ derived from lactate to total CO₂ production rates were less than 15% except one case These facts showed that oxidation of lactate into CO₂ was greatly inhibited in tumor tissues. 3. Respiratory CO₂ yields from C-1 carbon of lactate in various cancer tissues were mean of 77.7% of total CO₂ yield from lactate and CO₂ yields from C-2 and C-3 carbon of lactate were mean of 9.1% and 12.6% respectively. These facts showed that carboxyl carbon of lactate oxidized more easily than α and β carbon of lactate. 4. In 10 kinds of cancer tissues, fractions of disappeared lacteate from media into CO₂ and pyruvate, which expressed as RLD CO₂ and RLDpy respectively, were about 5% in except 3 cases and less than 3% except one case. These fact showed that almost of disappeared lactate from media were degraded into compounds other than CO₂ and pyruvate. From the above date, it was suggested that in the oxidative pathway of lactate in cancer tissues CO₂ was easily Produced from carboxyl carbon of lactate by oxidative decarboxylation as in the normal tissue, and further oxidation of 2 carbon unit via TCA cycle was inhibited.