We studied the inhibitory effect on the PCA reaction of BSA-rabbit anti-BSA (BSA-anti-BSA) system in CP-sensitized and CP-unsensitized guinea pigs by the oral administration of C. equi. Strain Ko-85 (Ko-85) cells. The results were as follows: 1) Guinea pigs were sensitized by intraperitoneal injection with the CP. Nine days later, the animals were administered with an oral dose of 80 mg (W.W.) of Ko-85 cells. The animals were submitted daily to PCA reaction tests using BSA and rabbit anti-BSA serum. The inhibition of PCA reaction of BSA-anti-BSA system in CP-sensitized guinea pigs was shown from 4th day to 10th day after administration of Ko-85 cells. The PCA reaction of 9th day was recognized strong inhibition by the oral administration of Ko-85 cells. 2) CP-unsensitized guinea pigs were given an oral dose of 80 mg of Ko-85 cells, and the animals were submitted every other day to PCA reaction tests of the BSA-anti-BSA system. PCA elicited in these animals on the 9th, 15th and 17th days Ko-85 administration were inhibited compared with those in the control animals (which were not given an oral dose of Ko-85 cells). The strongest inhibition was shown in animals challenged on the 15th day after Ko-85 administration. The inhibitory effect of PCA reaction of BSA-anti-BSA system by the oral administration of an oral dose 80 mg (W.W.) of Ko-85 cells was recognized in CP-sensitized and CP-unsensitized guinea pigs.