C3H/He and Balb/c mice bearing SCC VII and EMT6/KU tumors, respectively, recieved continuous administration of 5-bromo-2'-deoxyuridine (BrdU) for 5 days using implanted mini-osmotic pumps to label all proliferating (P) cells. Nicotinamide was administered intraperitoneally before cisplatin injection and/or tumors were locally heated at 40°C for 60 min immediately after cisplatin injection. The tumors were then excised, minced and trypsinized. The tumor cell suspensions were incubated with cytochalasin-B (a cytokinesis-blocker), and the micronucleus (MN) frequency in cells without BrdU labeling (quiescent (Q) cells) was determined using immunofluorescence staining for BrdU. The MN frequency in total (P + Q) tumor cells was determined from tumors that had not been pretreated with BrdU labeling. The sensitivity to cisplatin was evaluated in terms of the frequency of induced micronuclei in binuclear tumor cells (MN frequency). In both tumor systems, the MN frequency in Q cells was lower than that in the total cell population. Nicotinamide treatment elevated the MN frequency in total SCC VII cells. Mild heating raised the MN frequency more markedly in Q cells than in total cells. The combination of nicotinamide and mild heat treatment increased the MN frequency more markedly than either treatment alone. In total SCC VII cells, nicotinamide increased 195mPt-cisplatin uptake. Mild heating elevated 195mPt-cisplatin uptake in total EMT6/KU cells. Cisplatin-sensitivity of Q cells was lower than that of total cells in both tumor systems. Nicotinamide sensitized tumor cells including a large acutely hypoxic fraction, such as those of SCC VII tumors, through inhibition of the fluctuations in tumor blood flow. Tumor cells including a large chronically hypoxic fraction such as Q cells were thought to be sensitized by mild heating through an increase in tumor blood flow.