The growth of transplantable tumors, both in the solid and ascites form, was associated with a concomitant elevation of plasma hemopexin (HPX). A study of the dynamics of HPX concentrations in plasma, tumor and urine of normal and tumor-bearing mice demonstrated that HPX elevation in the plasma did not result from a delayed clearance from the circulation or body. Neither the plasma disappearance curve of i.v. (125I)HPX nor the urinary excretion of its metabolites was affected by the presence of the tumors. A body half life of about 8 h was found for both tumors-bearing and control mice. It was calculated that the presence of tumors caused a 9- to 18-fold increase of HPX concentration in the animal, which was probably the result of an accelerated synthesis. Some accumulation of HPX was found in solid tumors, both by traces of radioiodinated HPX and quantitative determination of endogenous HPX. In the case of ascites tumor, no HPX could be detected in the tumor cells.