Leucotrofina administration to chickens of two different ages (2,5 IU/chicken to 3 days old chickens, and 5 IU/individual to 21 days old chickens), associated with an antigen (1 / 1000 Bovine Serum Albumin) (0,25 ml, respectively 0.5 ml/animal in young and in older chickens) affected several blood serum parameters as: the antibodies concentration, the gamma-globulins, the total proteins, and the nitrogen of the free amino-acids content in a manner corresponding to the experimental model and to age the of the animals.