Antiserum to wound-tumor virus (WTV) with a titer of 1 10,240 was prepared by injecting into rabbits WTV purified by steps including rate and equilibrium zonal centrifugation. When crude globulins were precipitated by ammonium sulfate solution at pH 7.0 and conjugated with crystalline fluorescein isothiocynate (FITC) by a dialysis technique, the recovery of antibody was about 50% as determined by precipitin ring test. By contrast, if the pH of the ammonium sulfate solution was not adjusted and the antiserum was conjugated with FITC by the method used previously, the recovery of antibody was only about 12.5%. (Conjugates prepared by the earlier method are designated A and those by the dialysis method D.) Also, when D conjugates were used to detect WTV antigens in the smears of exposed leafhoppers, much brighter specific and much less nonspecific fluorescence was observed than with A conjugates. In experiments correlating results of smear tests of vectors and their transmission of virus to plants, 107 smears of exposed leafhoppers were stained with the D conjugates. Of these, 76 were positive and 24 negative both by smear test and transmission. Seven leafhoppers were positive by smear test but did not transmit WTV in the transmission tests. On the other hand, there were no leafhoppers which were positive by transmission but negative by smear test. Four days of acquisition feeding and 22 days of incubation at 27.5° yielded as high a proportion of viruliferous leafhoppers as longer periods. The probability of susceptible insects acquiring the virus by a 24-hour acquisition feed at 27.5° was 0.55. WTV could be detected earlier by smear test than by transmission. A method of scoring smears of leafhoppers in ultraviolet (UV) light without using a microscope is described.