Our previous articles in this series described the production of five monoclonal antibodies (SA1-5) that bind to adrenal chromaffin cells and to cells in embryonic sympathetic ganglia and adrenal primordia (Carnahan and Patterson, 1991), and the downregulation of the sympathoadrenal (SA) antigens in vivo as neuronal markers begin to be expressed (Anderson et al., 1991). These results support the hypothesis that sympathetic neurons and adrenal chromaffin cells are derived from a common embryonic progenitor that displays both neuron- and chromaffin cell-specific markers. We have taken advantage of the fact that at least some of the SA antigens are expressed on the cell surface to isolate SA+ cells from embryonic day 14.5 rat superior cervical, sympathetic ganglia by fluorescence-activated cell sorting. This population of cells is significantly enriched in the expression of markers (tyrosine hydroxylase and neurofilament) found in the putative progenitors in situ. Growth in glucocorticoid maintains the expression of the SA antigens in the sorted cells and induces the chromaffin cell marker enzyme phenylethanolamine N-methyl transferase. In contrast, growth of the sorted cells in basic fibroblast growth factor, NGF, and insulin results in the rapid loss of SA 1 expression and the outgrowth of neurites. The ability to manipulate the fate of the SA+ cells in vitro confirms the suggestion from the in vivo observations that the SA+ cells in the ganglia are at least bipotential progenitors, capable of differentiating along the chromaffin or neuronal pathways.