Antisera from cats naturally exposed and resistant to feline leukemia virus lysed feline lymphoma cells in vitro with cat complement. Lymphoma cell lysis was detected by 51Cr release and by viable cell counts with phase-contrast microscopy. The rate of target cell lysis varied according to the nature of the antiserum and the origin of the complement used. Cat alloantisera, raised by immunization with target cells, lysed lymphoma cells rapidly with cat complement, but immune sera from cats naturally infected with feline leukemia virus caused slow lysis requiring up to 20 hr to reach maximum effect. Slow lysis of lymphoma cells in homologous serum mixtures was complement dependent, specific for leukemia virus-infected cells, and was found to occur in cat-immune whole serum. When 143 cat sera were examined for presence or absence of lytic antibody by 51Cr release and for antibody to feline oncornavirus-associated cell membrane antigen by immunofluorescence, the qualitative correlation obtained varied according to the complement used in the 51Cr release assay. With heterologous absorbed complement sources the correlation was 79% with guinea pig and 65% with rabbit; with homologous complement the correlation was 92%. The close association seen between high antibody levels and tumor resistance in this outbred species under natural conditions serves to emphasize the potential significance of these observations.

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This work was supported by Grant 6981-1A from the Max C. Fleischmann Foundation and Contract NO1-cb-64001 from the National Cancer Institute. This paper is dedicated to the late Dr. E. P. Adams.

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