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During a previous study of African horse sickness virus (AHSV) ( l ) , the plaquing efficiency of this virus under agar overlay was compared with the plaquing efficiency under methylcellulose (2) and starch gel (3). The low efficiency of plating under agar that has been observed for several viruses (4) was also noticed for AHSV. However, a marked increase in plaque number was observed when methylcellulose or starch gel were substituted for agar. Because of the technical difficulties in the preparation of the two latter overlays, .a substance that was simpler to handle was sought as a substitute for agar. This report describes the successful development of an overlay employing tragacanth gum and reports the results of a comparative study of plaque formation under tragacanth and agar with foot and mouth disease (FMDV) , vesicular stomatitis virus (VSV) , pseudorabies virus, AHSV, and measles virus.Materials and Methods. Cell cultures. Primary chicken embryo cells (CC) and the stable monkey kidney cell line (MS) were grown in 2-02 bottles in Earle's solution containing 0.5% lactalbumin hydrolyzate, 5% yeast extract (Difco), 10% calf serum, and 100 units of penicillin and 50 pg of streptomycin per ml. The baby hamster kidney cell line, BHK21 ( 5 ) , and the porcine kidney cell line, BD (6), were also grown in 2 oz bottles in Hanks' balanced salt solution containing the same amounts of lactalbumin hydrolyzate, yeast extract, calf serum, and an-*

SUMMARYSmall concentrations of actinomycin D increased the yields of both virulent and attenuated strains of measles virus in cultures of the stable simian BSC-I cell line by as much as five-to tenfold. The EDMONSTON virulent strain of measles virus did not induce detectable levels of interferon in these cells but interferon was induced by the SCHW~u'Z vaccine (attenuated) strain of virus over a wide range of virus input. Actinomycin D suppressed the induction of interferon in BSC-I cells. Interferon produced in BSC-I cells by the attenuated virus was able to protect both these cells and Vero (another stable line of green monkey kidney) cells against vesicular stomatitis virus.Plaques produced by the virulent and attenuated virus in Vero cells were approximately the same size, in contrast to variations observed in BSC-I cells. This was correlated with the failure of the Veto cells to produce interferon when inoculated with either the virulent or attenuated strain of virus and the failure of actinomycin D to affect yields of infectious virus. The results suggest that the effect of actinomycin D on virus yields and on plaque size in BSC-I cells is a result of inhibition of induction of interferon.

Three cytopathic strains of subacute sclerosing panencephalitis (SSPE) virus were isolated from brain biopsies of three patients. These strains were isolated and maintained by cocultivation of infected brain cells with fresh Vero cells. The biological characteristics of two strains were studied. It was found that these strains remain cell-associated after repeated cocultivations with Vero cells and produce plaques under fluid medium or traga-canth overlay. The correlation with measles virus was demonstrated by the plaque reduction test as well as by the immunofluorescence test. Large numbers of nucleocapsids were observed in the cytoplasm of infected cells but none in nuclei. Intracerebral inoculation of monkeys, adult guinea pigs, newborn and adult hamsters or mice was followed by acute encephalitis and death.

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