When MH2 virus is assayed on the chorioallantoic membrane the result depends very largely on the age of the embryo, the pock count on membranes of embryos aged 10 days being 2½ times that on those 8 days old. This was always much less than that determined in the same strain of day old chicks by the limiting dilution method. Slight adaptation of the virus to egg passage was found but this was lost when the virus was transmitted through chicks. Non reactor eggs were few, and the number decreased markedly with the increasing embryo age, to vanish at age 11 days and over. This work was done while on a grant from the Government of the Federation of Malaya. I am grateful to Professor C. H. Waddington for laboratory facilities and to Dr. J. G. Carr for valuable advice. I am also grateful for the material and facilities provided by the British Empire Cancer Campaign Unit at the A.R.C. Poultry Research Centre. © 1959, The British Empire Cancer Campaign for Research. All rights reserved.