Alveolar macrophages from eight rabbits, exposed for about 1 month (5 days/week, 6 hr/day) to an aerosol of nickel chloride, 0.3 mg/m3 (as Ni), were studied. The number of macrophages in the lavage fluid and the variance of the cell diameter increased. The macrophages contained laminated structures and most cells had an active cell surface. A few macrophages had a large number of laminated structures and a smooth cell surface. The capacity of the macrophages to reduce nitroblue tetrazolium (NBT) tended to be increased at rest and was significantly increased after stimulation with Escherichia coli. The bactericidal capacity of the macrophages was decreased. The effects were similar to those earlier described after exposure of rabbits for 1 month to about 1 mg/m3 of metallic nickel dust. After exposure both to metallic and soluble nickel the effects are probably caused by an increased amount of surfactant produced by the type II cells in response to nickel ions.