"Activating" and "precipitating" antibodies elicited by Escherichia coli beta-D-galactosidase can be isolated by assortment of the respective antibody-forming cells in vitro, showing that the two antibodies have different specificities. Both populations also segregate in vivo and are synthesized asynchronously. The interplay of different antibody species with the same immunogenic molecule and the role of these expected to be complex interactions in the regulation of the immune response is an area for further investigation.