Summary: Ability to produce precipitin was studied in newly hatched chicks and chicks of various ages up to 12 weeks. The interfacial method was used. Newly hatched chicks gave low-titered antisera to bovine serum; slightly over one-half showed no precipitin with the dilutions (lowest 1:12.5) tried. There was a gradual increase in production-ability up to 4 weeks of age. The average titer given by the sera of 5-week-old chicks showed a remarkable increase over that of the 4-week-old ones. This increase could, in part, be attributed to the fact that almost all birds responded at 5 weeks of age. Chickens of 6 to 12 weeks of age gave titers similar to the 5-week-old ones indicating that serological “maturity” was reached at 5 weeks as far as precipitins against serum-proteins were concerned.