Abstract: A germicidal air purifier for trapping and destroying airborne microorganisms is disclosed. The air purifier includes an ultraviolet radiation source and a juxtaposed filter medium. A one of the ultraviolet radiation source and the filter medium is fixed and the other is displaceable so that at least an upstream side of the filter medium is systematically exposed to germicidal levels of radiation. In a first preferred embodiment, a fixed ultraviolet lamp irradiates a cylindrical air filter which is rotated on its longitudinal axis in close proximity to the lamp so that the upstream side of the filter is systematically irradiated. In a second preferred embodiment, a radiant lamp fixture is moved reciprocally across an upstream side of a planar filter to systematically irradiate the filter. In a third preferred embodiment, a radiant lamp fixture is rotated about an axis which is orthogonal to its longitudinal midpoint so that a circular area of a planar filter is irradiated.