An attempt is made to summarize and synthesize salient points from the conference. Considering the immunoglobulin-synthesizing and hemoglobin-synthesizing cell systems in parallel, first attention is given to aspects of cellular differentiation that deal with specialization in the formation of the predominant molecular products, the immunoglobulins and the hemoglobins. Primary structural considerations point to similar genetic mechanisms as the basis for phylogenetic diversity and similarities within each of the two classes of macromolecules, and to similar restrictions on the sorts of amino acid substitutions that can be tolerated while retaining the funcional integrity of the molecules. In the immunoglobulins, the appearance of "variable regions" in the component polypeptide chains, presumably associated with the diverse specificities required of these molecules in order that they may serve their functions, provides additional challenges for interpretation, and the significance of present knowledge in this context is evaluated. Other aspects of cellular differentiation are approached by way of the developmental shifts in molecular products found in both systems, and the explanations that have been suggested for these shifts. A striking difference in the two systems, the phenomenon of allele exclusion in immunoglobulin control, is discussed. The review concludes with discussions of more complicated aspects of differentiation in these cell series: the role of hormones and other humoral substances, particularly with regard to the immunoglobulin-synthesizing system, deriving from the thymus; parallels among cell-lineage patterns in the various hematopoietic series; and evidences of the causes of senescence in the immunoglobulinsynthesizing system.