alpha 1-antitrypsin (alpha 1-A.T.) phenotypes were determined in 55 patients with rheumatoid arthritis (R.A.), 33 patients with R.A. and either obstructive airways disease or recurrent chest infections, 49 patients with fibrosing alveolitis (F.A.), 22 patients with R.A. and F.A., and 200 healthy controls. A highly significant increase in the frequency of MZ phenotype was found among patients with F.A., both with and without R.A. Patients with R.A. alone had a normal distribution of phenotypes. Inherited modification of immune function may predispose to F.A. Alternatively, lowered levels of alpha 1-A.T. associated with non-M phenotypes may predispose to tissue damage with subsequent fibrosis.