Preliminary results are presented after measuring Pregnancy alpha 2 glycoprotein (P.A.G.) in a series of healthy males and those with cancer of the prostate, some of whom were being treated with oestrogens. Serum P.A.G. levels were measured in 21 patients with cancer of the prostate to observe any changes occurring during treatment with oestrogens. There was no significant difference between the P.A.G levels in healthy males and those with untreated prostatic cancer. Treatment however causes increased P.A.G. levels with wide individual variations. There is no apparent relationship between P.A.G. levels and the tumor stage, or efficacy of treatment. A relationship does exist however between the curves of serum P.A.G. levels in pregnant women and patients with prostatic cancer treated with oestrogens. Until proved otherwise, these measurements are of no practical value in patients with prostatic cancer, and future confirmation of these results by the study of a larger number of cases would be of value only in that they avoid other teams from repeating the same investigations.