RGS1 and RGS2 are members of a new class of regulators of G-protein signaling identified by their selective mRNA expression either in phorbol ester (TPA)-stimulated human B lymphocytes (RGS1/1R20/BL34) or in blood mononuclear cells treated with the T-cell lectin concanavalin A (ConA) and cycloheximide (RGS2/G0S8). The RGS1 gene shows low basal mRNA expression in freshly purified blood mononuclear cells, which increases upon incubation for a day. In contrast, RGS2 initially shows high basal levels of mRNA expression, which subsequently decrease. Expression of both genes increases in response to ConA, with RGS2 mRNA levels increasing briskly to a maximum between 0.5 and 1 hr and decreasing to baseline by 6 hr, whereas the RGS1 mRNA increase is delayed reaching a maximum between 1 and 2 hr. RGS1 mRNA levels increase much more in response to a protein kinase C activator (TPA), than to a calcium ionophore (ionomycin), whereas the opposite is true for RGS2. We suggest that ConA elevates RGS2 on the basis of its ability to increase intracellular calcium, and that RGS2 may be involved in the regulation of intracellular calcium. The distinction between RGS1 and RGS2 is further emphasized by studies indicating that recombinant RGS2 does not bind in vitro to two members of the G₁ subfamily of G-protein α-subunits for which recombinant RGS1 has high affinity.