Three human lung small cell carcinoma (SCC) xenografts serially transplanted into BALB/c nude mice were used for cell kinetic analysis. SCC strains, Lu-24, Lu-130 and Lu-134, were inoculated into the backs of nude mice, and when each tumor reached more than 300 mg, 50 microCi of 3H-thymidine per mouse was administered ip. The percentage labeled mitosis curve was obtained from the autoradiographic specimens which were labeled by the pulse-chase method. Cell cycle phase, growth fraction (GF) and cell loss factor (CL) were assessed by the methods of Quastler, Fujita and Steel, respectively. These cell kinetic parameters were compared with those of six control human gastrocolic and breast carcinoma xenografts which were previously reported by us. It was noticed that the cell cycle times (Tc) of SCC were statistically shorter than those of controls and this short Tc was found to be dependent on their short post-mitotic resting phases. GFs and labeling indices of SCC were observed to be statistically lower than those of controls, suggesting an incomplete adaptation of SCC xenografts to the host nude mice. Whereas some modifications by the host mice on the cell kinetics were supposed, the characteristics of SCC cell kinetics were thought to be essentially preserved in nude mice and these kinetic parameters were observed to be stable throughout the serial transfers. Accordingly, the SCC xenograft-nude mouse system was considered to be useful as an experimental therapeutic model of human lung small cell carcinomas.