Summary: An egg-adapted strain of herpes simplex virus was used in a study of the neutralizing activity of human sera in the embryonated egg. The neutralization test employing lethal endpoints after the injection of virus-serum mixtures into the yolk sac was found to be simpler and more reliable than that employing the reduction of pock count on the chorioallantoic membrane. Undiluted serum neutralized disproportionately more virus than diluted serum. The neutralizing capacity of dilute human sera was greatly enhanced by incubating virus-serum mixtures for 1 hour at 37 C prior to injection. The procedure increased the neutralization index of different sera in varying degree and thus permitted their distinction.