SINCE Seal et al.1 showed that, in the human plasma, the high affinity binding of progesterone is due to the corticosteroid binding globulin (CBG), it has been assumed by most investigators that this is the case in all species and in all conditions. Diamond et al.2, however, have found that in the pregnant guinea-pig the binding of progesterone is raised by a factor of 100 above that in the non-pregnant animal, whereas cortisol binding is only raised by a factor of 4. In this report, we show that, as Diamond et al. suggest2, there is a high affinity progesterone binding plasma protein (PBP) different from CBG and present in the pregnant guinea-pig.