After the isolation from oestrogen-induced prolactin-producing tumours in mice of a protein (mouse pituitary tumour protein = MPTP), a radioimmunoassay has been developed based on 125I-labelled MPTP (labelled by the Chloramine-T method of Greenwood, Hunter & Glover, 1963) and a rabbit antiserum to MPTP suitable for the estimation of prolactin in pituitary tissue homogenates and mouse plasma (H. G. Kwa, F. Verhofstad & E. M. van der Bent, unpublished observations). Parallel inhibition curves obtained by (1) unlabelled MPTP, (2) mouse anterior lobe tissue and (3) plasma from pseudopregnant mice suggested that MPTP was prolactin. Results are now given for plasma levels of prolactin, expressed in mμg. of the MPTP standard, in female mice during the oestrous cycle and in μg. at various time intervals after implantation of isologous pituitary tissue under the renal capsule. Blood samples were collected under light ether anaesthesia from the occular venous plexus, and the plasma