Antigen mediated nistamine release from RBL-2H3 cells is associated with substantial hydrolysis of membrane inositiol phospholipids and an increase in cytosol Ca2+ (Ca signal). As reported last year, studies with most variants of the RBL-2H3 cells revealed a correlation between hydrolysis of inositol phospholipids, Ca signal and histamine secretion in response to antigen stimulation. one variant, TG-1B3, however, showed no Ca signal as measured by quin-2, fura-2 and 45Ca2+-uptake but still showed modest hydrolysis of inositol phospholipid and histamine release. Several variants were completely unresponsive to antigen but did release histamine when challenged with a combination of ionophore and phorbol ester. Since these variants possessed normal numbers of receptors for IgE, and had abundant phospholipase C activity, the possibility of defective coupling between receptor aggregation and activation of phospholipase C was investigated. One variant BUDR-2B1, showed no or very little phosphoinositide hydrolysis in response to stimulants of GTP-regulatory proteins such as sodium fluoride in intact cells and GTP S in permeabilized cells. All other unresponsive variants did respond to these stimulants. In preliminary studies, the isolation, purification and hybridization of mRNA with specific radiolabeled probes for two GTP-regulatory proteins, Gi and Go, revealed that both the variants and the parental 2H3-cells transcribed message for Go but not for Gi.