1. By means of a quantitative immunoelectrophoretic technique, serum protein changes have been followed in seven patients before and during the week following inguinal herniorrhaphy, and in a single case of influenza.
2. The twenty proteins studied are grouped according to whether they rise (orosomucoid, α1-antitrypsin, Gc-globulin, caeruloplasmin, haptoglobin, β1A–C-globulin and proteins numbered 45, 99, 101 and X); fall (pre-albumin, α2HS-glycoprotein and protein No. 9); or show no significant change (α2-macroglobulin, haemopexin). α1-Lipoprotein fell in the case of influenza but did not change in the hernia patients; α1-easily-precipitable-glycoprotein increased in the hernia cases only.
3. The nature of the ‘acute phase reaction’ and the factors which elicit it are discussed.