200 3 to 5-weeks-old mice were treated with subcut. injns. of crystalline thyroxin 4 days before the inoculation with the rodent MM poliomyelitis virus so that the height of the thyroxin effect would coincide with the end of the incubation period. The single dose of thyroxin varied from 0.25y to 100y. No dose of thyroxin has been found which could protect the animals to a significant degree. No change in the length of the incubation period and no significant difference in the mortality between thyroxin treated and untreated animals could be observed.