Conditions have been established to permit poly-L-lysine (PLL)-attached cell monolayers to adsorb specific antibody-forming cell precursors (AFCP) from immune spleen cell populations as a negative selection technique. In a model system, sheep red blood cell (SRBC) immune spleen cells were adsorbed onto SRBC monolayers and the cells remaining non-adherent transferred to irradiated recipients. Subsequent to SRBC immunization, these recipients showed up to 95% reduction in splenic anti-SRBC plaque-forming cells (PFC) compared with recipients of control cell populations which had not been exposed to a SRBC monolayer. The depletion observed was shown to be antigen-specific and the extent of depletion comparable with that attained by removal of anti-SRBC AFCP as rosettes. The depletion of AFCP directed against mouse (EL4) and human (QIMR-WIL) leucocyte antigens was then examined. Mixtures of spleen cells immune to EL4 and WIL cells were adsorbed onto PLL-attached monolayers of EL4 or WIL cells, and spleen cells remaining non-adherent transferred to irradiated recipients which were then immunized with either of these cell types. Analysis of recipient serum samples by determination of antibody titre and by immunoblotting indicated that the response to the cell type used in the adsorbing monolayer had been specifically depleted while the response to the other cell type remained unchanged or only slightly impaired. The maximum level of depletion occurred on Day 7 after transfer and declined thereafter. The application of this procedure to improve the frequency of production of spleen cell hybridomas directed against leucocyte differentiation antigens is discussed.