The exposure of infected monolayers to increased temperatures during the early part of the incubation period was found markedly to inhibit plaque formation by viruses that replicate poorly at 40°, without reducing the number of plaques produced by strains able to grow at this temperature. This observation was used as the basis for the development of a sensitive test for the “rct” marker. The test, which has been termed the limited thermal exposure test, was applied to numerous test strains. The results were found to agree well with those obtained using other tests of the “rct character.” Inhibition of plaque formation by attenuated strains also occurred at 39°, a result indicating that the LTE test is less sensitive to variations in incubation temperature than other tests. Following the exposure of infected monolayers to increased temperatures for short periods, virulent, rct 40+ strains frequently showed an increase in plaque counts.