The FDA states, "Foods and drinks should not be heated on melamine-based dinnerware in microwave ovens." When melamine comes in contact with acidic foods and is heated to a temperature of 160 degrees fahrenheit or higher, the chemical compounds can leach out into food.
The Natural Resources Defense Council advises against heating food in anything other than glass or ceramic containers in the microwave. Even plastic containers labeled "microwave-safe" can leach micro-amounts of chemicals into foods. The term "microwave-safe" means that the amount leached is at levels far below those shown to cause health effects, and that the container does not melt or crack in the microwave.