Studies on a naturally occurring human antibody active against mouse landschutz ascites tumor cells: I. Cytotoxicity assay system and quantitation in normal sera

1. A quantitative cytolytic assay system was developed for the titration of the natural cytotoxin of human serum (NHS) which is active against Landschutz ascites tumor cells. The assay system consists of NHS₅₂ or NHS as a source of antibody and human umbilical cord serum as a source of complement. 2. Von Krogh plots of the cytotoxin and complement titration data gave 1/n slope values which were comparable to those obtained in the literature for acquired hemolytic and cytolytic immune systems. The values for the cytotoxin were compatible with those obtained with IgM antibodies. 3. The level of this natural antibody in 101 individual sera was examined. Titers ranging from 10 to about 600 LD₅₀ units/ml were found. On the average 93.4 units/ml were present. 4. There was a relationship between blood group and antibody titer. The mean titers of groups O and A individuals were higher than those of groups B and AB individuals. This would indicate an antigenic cross‐reactivity between the B blood group antigen and antigen (s) on the ascites tumor cell. 5. Guinea‐pig serum could replace human complement in the titration of the natural cytotoxic antibody but was much less effective than human complement.