The Bureau of Mines laboratory tested the response of a commercially available real-time aerosol monitor (GCA RAM-I) to various dusts. Monitor measurements were recorded, averaged, and compared with simultaneous gravimetric measurements of each test dust. Tests usually lasted several hours. The test dusts of various particle size distributions used included coal, limestone, and a commercially available test dust. For each particular dust, the monitor response was linear and correlated well with mass concentration over the range of about 0.5 to 10 mg/ma3 The monitor can estimate a 2.0-mg/m3 respirable coal dust concentration within as little as ± 6 pet with 95 pet confidence. The monitor must, however, be calibrated with the dust to be measured because the instrument response is affected by the type of dust particle. The average monitor response to a mass concentration of coal dust was approximately twice the average monitor response to the same mass concentration of limestone dust.