On July 10, 1984, the Environmental Protection Agency published in the Federal Register a ruling which authorizes the use of polychlorobiphenyls (PCB's) (i.e., aroclors) as a mounting medium for microscopy and as an immersion medium in fluorescence microscopy. The important properties for a mounting medium are discussed, as well as how to choose when it is appropriate to use an aroclor (in particular numbers 5442 and 1260) and the alternatives available. Properties of some of the aroclors are presented including health and environmental effects. A step-by-step procedure for their proper use is provided including transfer between containers, slide preparation, clean up, and waste disposal. -- AATA