The sequence dependency of the antitumor effect of etoposide and cytarabine (ara-C) was investigated against the L1210 ascites tumor in BDF1 mice. Etoposide (7.5mg/kg or 15mg/kg) and ara-C (25mg/kg or 500mg/kg) were administered intraperitoneally on days 1,4, and 7 after inoculation of L1210 cells with or without a time interval of 3 or 6h. Simultaneous administration of etoposide and ara-C produced a 70% cure rate. At every dosage examined, pretreatment with etoposide given 6h before ara-C was the most effective antitumor schedule in L1210 leukemia. At 1h after injection of ara-C, 3h and 6h pretreatment with etoposide 15mg/kg increased ara-C incorporation to more than 200% as compared with that of ara-C given alone. Simultaneous administration of etoposide, however, decreased ara-C incorporation to 33% of that of ara-C alone. Deoxycytidine kinase (dCK) is a rate-limiting enzyme for the activation of ara-C. We demonstrated that dCK activity was increased within 1h after exposure to etoposide. Much more attention must be paid to the timing of the administration of etoposide in combination chemotherapy with etoposide and ara-C.