NIOSH recommends that employee exposure in the workplace to hydrazine, methylhydrazine, 1,1-dimethylhydrazine, 1,2-dimethylhydrazine, and phenylhydrazine, and their salts formed by addition with acids, such as sulfates, hydrochlorides, or hydrobromides, be controlled by adherence to the following sections. The standard is designed to protect the health and provide for the safety of employees for up to a 10-hour workshift, 40-hour workweek, over a working lifetime. Compliance with all sections of the standard should, as a minimum, substantially reduce the risk of cancer induced by these hydrazines and prevent other adverse effects, both acute and chronic, which could result from exposure in the workplace. Sufficient technology exists to permit compliance with the recommended standard. The employer should regard the recommended environmental limits as the upper boundaries of exposure and make every effort to maintain the exposure as low as is technically feasible. The standard will be subject to review and revision as necessary. The recommended standard is based on the conclusion that valid evidence of skin absorption, blood and liver effects, and tumor in induction in experimental animals by these hydrazines is relevant to human exposure. No demonstrably safe level of exposure is evident, and in view of the severity of the toxic effects, especially carcinogenicity, the limits of exposure recommended represent the lowest detectable concentrations. The environmental limits are likely to offer greater protection from nonneoplastic effects from some hydrazine compounds than from others. They assure protection to individual compounds only when skin absorption is prevented and they cannot be directly extrapolated to mixtures.