Airborne particles were collected in two small urban gardens along the roads with heavy traffic in Hong Kong. The chemical and physical properties of the particles of a diameter about 1 μm were analysed by a scanning electron microscope incorporated with a x-ray energy dispersive microanalyzer and an atomic absorption spectrophotometer. The cytotoxicities of particles were compared using an in vitro alveolar macrophage culture assay. It was found that both particles significantly increase (p < 0.05) the release of lactate dehydrogenase and acid phosphatase into the medium in addition to damage the surface morphology and intracellular structures of the macrophages to a variable extent after exposure to the particles. Copyright © 1987 Published by Elsevier B.V.