Comparison of three methods of fluorescent antibody staining revealed the complement staining procedure to be the best method available for the virus-antibody system under investigation. The procedure showed good correlation to results obtained in the in vitro complement fixation test and could be applied quantitatively for maximum specificity and sensitivity. Drying of infected cover slip cell cultures, prior to fixation, resulted in total conversion of existing N to H antigen, which is, nevertheless, typespecific. Investigation of intracellular synthesis of specific antigen, by the complement staining method, and simultaneous assay of production of mature virus, led to the conclusion that both begin at the same time, from 2 to 3 hr after infection depending on the multiplicity of infection employed. Near or at the completion of the infectious cycle, about 300 plaque forming units of virus were produced per fluorescent cell