Studies of the cell killing effectiveness of ftorafur were carried out against 5-fluorouracil (5-FU). It appeared that in divided doses ftorafur is both relatively less toxic in terms of LD50 and bone marrow stem cell killing and less effective against L1210 and a mouse osteosarcoma than 5-FU. Pharmacokinetic analysis revealed that upon repeated administration of 5-FU there is an increase in late drug serum levels and a decreased clearance of 5-FU from the serum. Repeated administration of ftorafur is not associated with significant differences in serum levels of 5-FU or ftorafur.