Tantalum, an experimental bronchographic material, may be retained in the lungs for a prolonged period following bronchography. The alveolar macrophage (AM) is a cell with potential for clearing tantalum particles from the airways. We studied the in vitro effects of tantalum oxide and two other particles, silica and latex, on rabbit AM viability and lysozyme release over 30 hours. Results indicate: 1) tantalum oxide, silica, and latex particles are ingested by rabbit AM in culture; 2) tantalum oxide and silica are both toxic to AM in vitro; and 3) tantalum oxide exerts its toxic effects less rapidly on AM than does silica. On the basis of these in vitro culture results we conclude that tantalum oxide may be toxic to alveolar macrophages in vivo. Delayed lung clearance of tantalum oxide particles may be due in part to their toxic effects on alveolar macrophages.