The purpose of this study was to increase the sensitivity of the staining reaction for the T antigen on the surface of neoplastic cells grown in vitro with the use of site-specific monoclonal antibodies (MAbs). The authors describe anti-peanut agglutinin (PNA) MAbs selected by screening the hybridomas with PNA and PNA bound to bovine serum albumin conjugated with the T antigen. The selected hybridomas (F2C8, F3D12, F3A5) were then grown in pristane-sensitized mice or in the Amicon Hollow Fiber System (F2C8). The affinity constant values for PNA were measured, and all the purified MAbs were tested on both native and denatured PNA, wheat germ agglutinin, concanavalin A, and ricin by using the immunoassay dot test and immunoblotting methods. Eleven different cell lines were stained with the three MAbs; similar results were obtained with F2C8 and F3D12. In each case the fluorescence, if present, was associated with the cell membrane, and the intensity of the staining was always stronger when the cells were incubated with the MAbs than when stained with fluorescein-labeled PNA. On the other hand, F3A5 failed to stain unfixed cells preincubated with PNA but stained the same cells after fixation, independently of the presence of PNA. One of the antibodies, F2C8, was used to stain histologic preparations from 27 cases of Hodgkin's disease and was compared with the anti-granulocyte antibody, Leu-M1, which has been used by numerous authors to identify the characteristic Reed-Sternberg cells. The results obtained were qualitatively similar; ie, F2C8 was at least as efficient as anti-Leu-M1 in its ability to stain the typical diagnostic cells in Hodgkin's disease.