Microwave-induced hyperthermia was focused locally on mammary adenocarcinoma implanted in C3H mice when the tumor reached 6 mm in diameter. In 54 treated mice, all tumors diminished in size after the 1st exposure (43.degree., 45 min) and disappeared completely following the 2nd treatment. No other effects from heat application were apparent. All mice in the microwave-treated group survived the whole period of observation (4 mo.) without any evidence of tumor, while 18 nontreated controls died within 4 wk after inoculation.