The Antigen L is made of the 10 p.100 TCA precipitable proteins that remained after 2 p.100 TCA proteins precipitation of a live BCG culture filtrate. It is 6 times more active in guinea pigs sensitized by living BCG than by inactivated BCG. The reference tuberculin (BR6) is the fraction retained by DEAE-Cellulose chromatography of the purified TP tuberculin described elsewhere (2). The kinetic study of BR6 using guinea pigs sensitized by live or inactivated BCG showed that between the 2nd and 4th month, BR6 had the same activity in both cases in respect to the diameter of the erythema reactions. Therefore, BR6 may serve as reference for the Antigen L between the 2nd and 4th month of sensitization.