The effect of progesterone on the secretion of protein by the magnum of 5-d-old, female chicks was determined. 2. The supernatant prepared by centrifuging an homogenate of the magnum at 105 000g was found, by immunodiffusion, to contain an antigenic component which precipitated the antisera for conalbumin 1, conalbumin 2 and ovalbumin after 5 d treatment with progesterone: there was no reaction to ovomucoid, lysozyme and avidin antisera. 3. Disc-electrophoresis of the homogenate revealed two bands at the site of ovalbumin. 4. Incorporation of 3H-lysine into the magnum proteins of progesterone-treated chicks did not differ from that of controls. 5. The secretion available in the magnum may be only a transudate from the serum and not a true secretory product. Progesterone behaved qualitatively as oestrogen in this study although the action is much less pronounced and was delayed.