An asbestos fiber counting trial has been conducted in 19 asbestos-cement board plants through-out Japan, based on the phase-contrast microscopic count on membrane filter samples. In the four working places, i.e., bag carrying-in, bag opening, board cutting and shipping, airborne chrysotile fiber counts in 1 cm³ of air, the geometric mean and standard deviation were 1.86(1.73), 3.87(3.01), 2.34(2.59) and 1.26(2.17) respectively. Mean size distribution (geometrical length) of the fiber in bag opening and board cutting were 8.91μm (σg 2.88) and 5.51μm (σg 1.22) respectively by measuring on the magnified photomicrographs. As the clearing agent of filters, dimethyl phthalate with diethyl oxalate containing dissolved filter material was used.