To examine the effect of various incubation temperatures on the apoptotic death of human peripheral blood mononuclear cells (PBMC), we incubated cells at 37 degrees C, 22 degrees C, and 4 degrees C for 1 and 24 hours. In addition, cells incubated at 4 degrees C for 3, 6, and 9 hours were rewarmed to 37 degrees C until a total incubation time of 24 hours was reached. The percentage of apoptotic cells was detected by a flow cytometric assay using propidium iodide staining. Incubation of PBMC at the above-mentioned temperatures for 1 hour did not affect the percentage of apoptotic cells. However, incubation at 4 degrees C for 24 hours resulted in the lowest percentage of apoptotic cells compared to those incubated at 22 degrees C and 37 degrees C. Rewarming of the cells to 37 degrees C increased the percentage of apoptotic cells to a level similar to that of the controls (incubated at 37 degrees C). Because PBMC are closely involved in the normal function of the immune system, the results of the study should be considered in cases in which these cells are exposed to various thermal conditions.