In view of the endemicity of smallpox in Madras and the poor take rate on revaccination with the lymph in current use, it was felt that the immunity of the general adult population might be relatively low. In an attempt to obtain some measure of this, sera were obtained from over 300 adults in Madras for the estimation of serum antibody. The results of neutralization tests against variola virus made on these sera showed that about 10% of those who had good scars from infant vaccination showed little or no antibody. A comparison of the antibody titres of adult sera showing good antibody levels when used undiluted was made with that of sera from smallpox convalescents. The antibody level in the latter sera was 20-100 times that of the sera of adult persons who showed good vaccination scars.