The blood plasma of Spermestes c. cucullatus (Swainson) analysed electrophoretically on agar shows seven fractions.Immunoelectrophoresis against an antispermestes rabbit serum reveals 16 fractions.Individual variation of the plasma protein fractions is very low, as is also the case for the mobility of the fractions. With age, the α-globulins disappear gradually.In the newly hatched bird, the protein variations are more pronounced than in adults. Most important are the lower relative content of albumin and a corresponding relatively higher content of β-and γ-globulins.There seems to exist a slight sexual dimorphism in albumin level, which is higher in males than in females.During the reproduction period, both sexes show a fall of the relative concentration of albumin.Injections of gonadotropine hormone produce an increase of the plasma-proteins level and a marked decrease of the relative concentration of albumin. The overall figure is comparable with that of birds in natural breeding condition.