The results of serum neutralizing tests with the viruses of pox, vaccinia, and virus III disease herewith reported generally agree and supplement the results of reinoculation experiments on immune rabbits reported in the previous paper (1). The finding that pox virus is neutralized by pox immune serum indicates that the refractory state of recovered pox rabbits to reinoculation with pox virus and the failure of recovered pox rabbits to contract a second pox infection after adequate exposure is to be explained upon the basis of the development of an active immunity. The failure of virus III immune serum to neutralize pox virus is in agreement with the previous conclusion drawn from the positive results of reinoculation and exposure experiments that there is no specific relationship between pox virus and virus III. Rabbits which had recovered from a pox infection were completely refractory to inoculation with dermo-(culture) vaccine, while rabbits which had recovered from vaccinia were partially refractory to inoculation with pox virus. Vaccine immune adult rabbits did not show any clinical evidence of pox upon exposure to florid cases, but young recently weaned vaccine immunes developed definite although comparatively mild pox infections. The serum neutralization tests showed that pox immune serum neutralized vaccine virus although the action was not complete as shown by the positive results obtained with high dilutions of serum; vaccine immune serum possessed some but comparatively slight neutralizing properties against pox virus. The results of the crossed inoculation and serum neutralizing experiments with pox and neurovaccine viruses resembled those obtained with pox and dermo-(culture) vaccine but the differences were less pronounced. The differences in virus neutralizing ability on the part of the three immune sera paralleled the differences in virus potency as indicated by the character of the local lesions at the site of injection and by the general character of the clinical manifestations of the infection. The potency or virulence of pox virus was much greater than that of neurovaccine and vastly greater than that of dermo-(culture) vaccine. The complete identity of pox and vaccine virus could not be established, but a definite relationship between them was demonstrated and this was shown to have an immunological basis. From a practical standpoint vaccination with vaccine virus as a prophylactic measure against rabbit pox was clearly indicated.