IgG and IgM antibodies became demonstrable by indirect immunofluorescence on days 7 to 9 of illness in 35 survivors of Crimean-Congo hemorrhagic fever. Maximum titers of antibody were usually attained in the second to third week of illness. Titers of IgM declined gradually thereafter and were low or negative by the fourth month. In some patients titers of IgG increased markedly between 2 and 4 months after onset of illness and remained readily demonstrable by indirect immunofluorescence 3 years after infection. Endogenous antibody response was demonstrated in only two of 15 patients who died of infection. Techniques for demonstrating antibody were (in order of decreasing sensitivity) enzyme-linked immunosorbent assay, reversed passive hemagglutination-inhibition, indirect immunofluorescence, fluorescent-focus reduction, complement-fixation, and immunodiffusion. Most patients developed relatively low levels of neutralizing antibodies (range, 1:8 to 1:32 by fluorescent-focus reduction tests), but some developed titers of 1:256 to 1:512. Plasma intended for therapeutic use should be selected on the basis of its neutralizing ability.