IN 1937 Dr. K. Brdi?ka followed up an earlier (1933) observation that the sulphydryl group in proteins causes a distinct catalytic effect on polarographic current-voltage curves, by applying it to diagnose cancer through the effect the disease has upon the proteins in the blood. The results were presented to the Czech Academy of Sciences (37, 17 ; 1937) ; but publication was delayed and it is only recently that the account has reached Great Britain. It was found that carcinomatic sera give definitely smaller polarographic effects than normal sera. The method consisted in taking 0.3 c.c. of centrifuged blood serum and adding 0.15 c.c. of normal potassium hydroxide solution and 0.3 c.c. of potassium iodoacetate (which reacts with the sulphydryl group in proteins). After standing an hour, amounts of 0.1 c.c. of the mixture are taken out at intervals and added to 5 c.c. of a cobalt chloride solution containing 8 x 10-4 molar cobalt chloride, 0.1 normal ammonium chloride and 0.1 normal ammonia. The polarographic curves of these solutions are recorded, using a dropping mercury cathode, the anode being the mercury layer below the solution. In this way 67 cases were examined ; 32 gave the abnormally low (carcinomatic) protein effect, in agreement with the clinical conclusions.