Summary and Conclusion A tissue culture tube overlay method for detecting virus growth and for performing titrations and serum neutralization tests with members of the arthropod-borne viruses has been described. This method was more consistent and more sensitive than cytopathogenic effects for detecting multiplication of certain arbor viruses in tissue culture. Plaque-forming unit endpoint titrations were generally higher than the cytopathogenic endpoint (TCD50) even when CPE was evident, or the endpoint (LD50) in infant mice inoculated intracerebrally. It is suggested that the tube overlay technic may be an aid in large scale screening of clinical or field specimens and in conducting serum neutralizing antibody surveys for many arthropod-borne viruses.