(1) Differential protein clearances have been measured in 20 patients suffering from toxaemia of pregnancy. The proteins measured were albumin, transferrin, IgG, ceruloplasmin, IgA, C′3 complement, α2-macroglobulin (α2M) and IgM. (2) Selectivity of proteinuria was high in 14 cases, where no protein molecule with a molar weight greater than 180,000 was found. (3) In 6 cases, selectivity was low with a high differential protein clearance of α2M. (4) IgM was not detected in the urine in any patient. C′3 was found in one patient’s urine only. Low selectivity was related to a total proteinuria of at least 6 g/day. The anomalies in differential clearances of the individual proteins are discussed. (5) Low selectivity and the presence of α2M in the urine were associated with a higher perinatal fetal mortality than in cases of high se