The fractional deposition in human subjects of natural, radioactive particles has been determined by use of simple apparatus. This apparatus consists of a modified filter respirator, rubber bag, sample filter holder, air flow meter, and air pump. A correction has been applied to deposition measurements to compensate for respirator "dead space." Preliminary measurements of the deposition of radon daughter products in four humans subjects breathing New York City air are in the range of 17.9-50.7 per cent.