t Ct/min with P3NS1 spent medium. § Supernatant 791T/36 Clone 3 was diluted 1:10 and absorbed with 108 cells/ml at room temperature for 2 h. These conditions were based upon those used successfully with a monoclonal antibody to a rat breast carcinoma, which has a titre against relevant target cells of 1/1000 (Gunn et al., 1980); this was also the titre empirically determined for 791T/ 36 Clone 3. Table III shows **data** from experiments in which the monoclonal antibody was (a) absorbed by 791T cells and tested against other cross-reactive target cells, (b) absorbed with other tumour cells and tested against 791T, or (c) absorbed with, and tested against cells other than 791T. Where the cross-tested absorbing cells or target cells were reactive with the antibody (Table II) reductions of bound ct/min of between 70% and 90% were obtained after absorption. When the antibody was absorbed with non-crossreactive cells (Mel 2a, Mel 57, PA-1) however, the reduction of activity against 791T cells was only 11-24%. These results support the specificity of the antibody demonstrable in direct tests.