The characteristics of memory B cell antibody-forming cell (AFC) progenitors from long-term hapten-primed CBA mice were investigated by using sedimentation velocity and buoyant density separation to isolate physically distinct B cell sub-sets. The isolated fractions were assayed by the adoptive immune response to NIP-POL antigen, under conditions where neither T cells nor other accessory cells were limiting the IgM or IgG AFC responses. The results were compared to previous studies on the IgM AFC-progenitors of unprimed adult mice. Splenic IgM and IgG memory AFC-progenitor activity was largely found among the typical B cells of slow to medium sedimentation rate, in contrast to the fastre sedimenting IgM AFC-progenitor activity of unprimed animals. Splenic IgM and IgG memory AFC-progenitor activity was found among the medium to light density cells, and so resembled by this parameter the IgM AFC-progenitor activity in unprimed animals. Thoracic duct lymphocytes from hapten-primed mice also exhibited memory IgM and IgG AFC-progenitor activity in the slow-medium sedimentation range. However, in contrast to spleen, the IgM and IgG memory AFC-progenitor activity in lymph was found among very dense B cells. Two physically distinct sub-populations of memory B cells have thus been identified, namely: i) small, medium-light density, presumably tissue-resident B lymphocytes found in spleen; and ii) small, dense, presumably recirculating B lymphocytes found in lymph. Both physical forms include IgM and IgG progenitors. Both forms are distinct from the larger, medium-light density "virgin" AFC-progenitors in the spleen of unprimed adult mice.