A test to sterilize pieces of cloths, which are the material of hospital white coats (doctors and nurses wears), by microwave irradiation in place of autoclaving was done using a commercial 2,450 MHz microwave oven. When pieces of cloths made of cotton (35%) and polyester (65%) were contaminated experimentally with Escherichia coli or Staphylococcus aureus and irradiated by microwave, the bacteria were killed quite rapidly according to almost first-order reaction kinetics. Only after a 20-sec irradiation, when the water content of cloths was decreased from the original 48% to 35%, the relative survivals of these bacteria fell to below 1% that of the non-irradiated control. The cloths were neither browned nor crisped, even after a 10-min irradiation of microwaves.