This study was carried out to investigate the effects of Nd:YAG laser-induced hyperthermia on murine F9 embryonal carcinoma cells in vitro using various power settings, temperatures, and exposure times. F9 cells were plated on gelatin-coated dishes, treated on the following day, and cultured overnight. The following day the killing efficiency of the treatments was estimated by staining the dishes or by labeling the cells with 3H-thymidine. A contact Nd:YAG laser with a frosted-end probe was used. After laser treatments at 39 degrees C, no significant changes were observed in the viability of the cells. Laser treatment at 43 degrees C killed F9 cells, and the effect was related to the power setting used. Using 6 W, the quantity of viable cells progressively decreased after 1-, 2-, and 5-min treatments, and no viable cells were found after a 10-min treatment. Using 10 W, approximately 10% of the cells survived a 1-min laser treatment, but all cells were killed after a 2-min treatment. In the control wells, heated in a water bath for up to 40 min, all cells regularly survived at 43 degrees C. There were much less viable cells in those laser-treated wells where the temperature exceeded 44 degrees C than in those where the temperature was kept at 44 degrees C. In conclusion, the tumoricidic effect of hyperthermia can be potentiated by the use of the contact Nd:YAG laser. At a set temperature the cell killing effect of laser treatment is dependent on the power used and the duration of the treatment.(ABSTRACT TRUNCATED AT 250 WORDS)