The Gas Proportional, Alpha, Beta-gamma Continuous Air Monitor was developed for immediate and simultaneous detection of long half-life radioactive airborne particulate collected on a large fixed filter media. The alpha and beta-gamma counting rates and the nullification or differentiation of these counting rates are indicated separately on linear count rate meters and strip chart recorders. Efficency of the alpha section was found to be 35 per cent and the beta section to be 20% of total disintegration from a flat plate standard. Sensitivity of the monitor at a flow rate of 35 cubic meters per hour is such that a level of less than 1 MPL may be detected within an hour under average operating conditions.