Concern over the alleged hazards from asbestos containing friction materials has led to a major effort in the search for alternative materials for asbestos. Claims of adequate technical performance of the non-asbestos friction products have been made, as well as guarded judgments. In all cases however, the asbestos-free label may have conveyed implications of innocuity with regard to the non-asbestos fibrous substitutes, with possible relaxation of safe working practices. In this paper, an overview of rapid screening biological assays for testing airborne particles is presented. A brief survey of the known biological effects of some asbestos substitute materials is presented, which points to the need for workplace control of all airborne respirable materials used or proposed in friction materials.