Serum alpha 2-macroglobulin concentrations were measured in 178 patients with emphysema and 115 control subjects of similar age and sex distribution. The study group included 59 PI type Z patients with alpha 1-antitrypsin deficiency, five with the rare alpha 1-antitrypsin null genotype (PI Q0 or --), and seven with alpha 1-antitrypsin deficiency of the rare PI types MmaltonZ or MduarteZ. Individuals with all types of alpha 1-antitrypsin deficiency were found to have significantly increased serum concentrations of alpha 2M (p less than 0.001). These increased concentrations were associated with all types of alpha 1-antitrypsin deficiency, not only with the PI type Z. The highest alpha 2-macroglobulin concentrations were found in the PI Q0 patients (5 with emphysema, 2 with no lung disease), and these patients had almost no circulating alpha 1-antitrypsin. Raised concentrations of serum alpha 2-macroglobulin were not due to emphysema: 86 patients with emphysema, of PI type M, and the normal control subjects had similar average concentrations of alpha 2-macroglobulin. One control subject with an average alpha 2-macroglobulin concentration of only 41% of normal was found.