The exact nature of the biological effects of microwaves is not completely understood. Evidence indicates that microwave energy can act as a 'stressor' agent, and has an effect on regulatory and integrative mechanisms of the body with resultant alteration in homeokinesis. Animals exposed to microwaves at specific frequencies and flux densities experience thermal stress. Duration of exposure, environmental temperature, and drugs that affect the central nervous system (CNS) and temperature regulation influence the response of animals. High ambient temperature exaggerates the thermal response, while exposure in a cold environment prolongs the time interval for an increase in body temperature. Microwave effects on hematopoiesis, thyroid function and interaction with ionizing radiation are discussed. Sufficient data are not available to establish a comprehensive safe level for microwave exposure because of microwave frequency related factors which affect biologic response. It is recommended that all microwave workers should undergo a thorough pre-employment and periodic medical examination. Persons with cardiovascular problems or lenticular defects should be considered as risks. Because of indication of bone marrow and thyroid sensitivity to microwaves, hematologic and thyroid function studies should be incorporated in the medical examination. An increase from the present maximum permissible exposure level of 10 mw/sq cm could introduce risks which might not be readily recognized.