A new test in mice is described in which luteotrophic substances induce mucification of the vagina. Evidence that vaginal mucification reflects pseudopregnancy is given by morphologic studies of the vagina, ovaries, and adrenal glands. Electrophoretic separation of components of a commercial preparation of human placental lactogen and heating of ovine pituitary prolactin failed to destroy their luteotrophic activity. Partial peptic hydrolysis of placental lactogen resulted in total loss of effect. The practicality of the mouse vaginal mucification test is demonstrated, so that it may serve as an adjunct to pigeon crop sac and mammary gland assays of different luteotrophic protein preparations.