In 1974 the California Department of Food and Agriculture (CDFA) adopted regulations pertaining to the safety of employees handling pesticides for agricultural uses and entering fields that might have pesticide residues. The regulations mandated that closed systems be developed for handling toxicity Category One liquid pesticides. While the regulations contained a general definition of closed systems, more specific criteria were not developed until some time later. The requirement for use of closed systems has been in place for several years, and debate continues over their usefulness. This paper reviews the development of California closed system criteria and the implementation problems encountered.