A subclone (clone 20) of chemically induced, murine colon adenocarcinoma with a potent ability to induce cachexia and another subclone (clone 5) without such an activity were transplanted to syngeneic mice (CDF1) and their tissue weights, blood components and cytokine levels in sera were compared. Mice transplanted with clone 20 showed a profound body-weight loss by 15 days after inoculation when the tumor accounted for less than 1% of the body weight, along with marked reduction of food and water intakes. Thereafter, they transiently gained in body weight with restoration of food and water intakes. Thus, the change in body weight was biphasic and not proportional to the tumor size. Body fat was lost preferentially, accompanied with a decrease in plasma triglyceride levels. The thymus contracted remarkably, and the peripheral lymphocyte count decreased extensively. Mice transplanted with clone 5, in contrast, did not show any of these changes characteristic of cachexia. Serum concentration of interleukin-6, which has been proposed as the principal inducer of cachexia in mice with colon 26, increased in mice with clone 5 to levels comparable to those in mice with clone 20. The changes in mice bearing clone 20 could not all be explained in terms of known biological activities of interleukin-6. Additional unknown factors, therefore, are presumed to contribute to cachexia in mice with clone 20. Identification of them should be helpful in the care of cachectic patients.