Increased chemotaxis toward activated serum was demonstrated by leukocytes from bone marrow, spleen, and peripheral blood of tumor-bearing mice as compared with those of normal mice. Increased chemotaxis was correlated with duration of tumor growth, which, in turn, was correlated with size of tumor. Upon surgical tumor removal, chemotaxis fell to normal levels unless metastasis had occurred, in which case, the extent of chemotaxis was correlated with the volume of the metastasis. Increased chemotaxis was seen in relation to the growth of a mammary adenocarcinoma as well as that of a chemically induced fibrosarcoma. The major chemotactant in serum was shown to be complement derived, and sera from tumor-bearing and normal animals were equally effective. The presence of a tumor resulted in an increase in the percentage of polymorphonuclear cells in the bone marrow. The numbers of cells which migrated were independent of the cellular composition of the bone marrow in normal mice. In contrast, a negative correlation was found between the percentage of lymphocytes present and the number of bone marrow cells which migrated toward activated serum when the cells originated in a tumor-bearing mouse. The data suggested the increased chemotaxis was a property of the cells rather than the soluble substances in the serum.