1. Proteinase inhibitors have been studied in whole serum by using a kinetic method that avoids potentially damaging protein separation procedures.
2. The α2-macroglobulin of an individual can be allocated unambiguously into one of seven categories according to the binding of trypsin to inhibitor in two kinetically apparent binding modes (β- and α-modes).
3. The distribution of α2-macroglobulin β:α ratios in a healthy adult population is defined, and shown to be independent of sex and age.
4. The distribution of β: α ratios in a group of patients with cryptogenic fibrosing alveolitis was found to be significantly different (P < 0.005) from the normal distribution.
5. Changes in the β: α ratio were noted in five of six patients with cryptogenic fibrosing alveolitis after treatment, but on no occasion when two healthy subjects were assessed a total of nine times.
6. The molecular interpretation and the possible importance of altered proteinase inhibition in inflammation and fibrosis are discussed.