It has been recently reported that fractionation of mouse spleen cells over columns of histamine-rabbit serum albumin-Sepharose results in a depletion of the number of anti-sheep erythrocyte plaque-forming cells (PFC) passing through the columns and in an elevation of the immune response potential of the same cell suspension when transferred into irradiated recipient mice. The objective of this study was to test the effects of such cell fractionation for other immunological functions, and to elucidate the cellular mechanisms involved.