Transferrin-neocarzinostatin (NCS) conjugates with differing molar ratios of drug to protein were synthesized and their intracellular metabolism was investigated. The conjugate mixtures of transferrin-NCS were separated by DEAE-Sephacel column chromatography. The separated molecular species were examined with respect to binding affinity to transferrin receptor, cytotoxicity and intracellular metabolism using the human leukemia cell line, K562. Transferrin-NCS conjugate is capable of binding to transferrin receptors specifically and its reactivity became weaker as the ratio of bound NCS to transferrin was increased. Transferrin-6NCS did not bind measurably to the receptor. On the other hand, the cytotoxicity was augmented when the number of NCS molecules bound per molecule of transferrin was increased to 4NCS/transferrin, while transferrin-5NCS and transferrin-6NCS species exhibited low activity. Examination of the kinetics of metabolism by pulse chase study using 125I-labeled ligand indicated that unconjugated transferrin and transferrin-NCS conjugates were internalized in similar ways, although the degradation of internalized conjugate was more marked in the case of transferrin-4NCS than transferrin-1NCS. Thus, the molar ratio of transferrin-drug conjugate could be optimized with respect to both the binding activity to receptor and the intracellular metabolic pathway.