SUMMARY A passive hemagglutination (HA) test using tanned horse red blood cells coated with partially purified fowlpox viral antigen, obtained by Genetron treatment, was used for titration of antibodies against fowlpox virus. HA antibodies were detected in sera of a few birds as early as 1 week after inoculation, though usually not until the third week after inoculation. HA antibodies were present in the sera of most of the birds up to week 15, when the birds were reinoculated. Agar gel precipitation test first detected precipitating antibodies between the 2nd and 5th weeks after primary inoculation, with persistence for 2 to 5 weeks.