Abstract
The improved overlay medium for plaque titrations was investigated on two strains of herpes simplex virus using GMK cells. The most favourable constituents of the overlay medium was as follows : 1.5% agar, 0.5% skim milk, 0.125% lactalbumin hydrolysate and 0.225% sodium bicarbonate in Earle's saline. The second overlay with neutral red was done after 3 days. In this system, HF and M strain developed the clear plaques with the size of 1 to 4.5 mm and 0.5 to 1.5 mm, respectively. There was a linear relationship between the virus concentration and the number of plaques. The plaque forming titer in GMK cells was as same as the pock forming titer on CAM, and higher than that in chick embryo cells. The possibility of inhibitory effect on the plaque formation of agar component could be ruled out by the experiment of adding protamin sulfate into overlay medium or of replacing the agar by the agarose.
