Abstract The effect of agents which alter the intracellular levels of cGMP as well as the enzymatic susceptibility of the Fc receptor on a murine lymphoblastoid cell line have been investigated. Incubation with dibutyryl cGMP showed an immediate increase in receptor activity. Imidazole, acetylcholine, and poly(I-C) also enhanced rosette formation but these effects were not evident until 24 hr. The Fc receptor has also been demonstrated to be sensitive to high concentrations of proteases which result in loss of receptor activity. Lower concentrations of proteases, however, as well as neuraminidase, enhanced rosette formation. Incubation with phospholipase A significantly reduced receptor activity while neither phospholipase C nor D had any effect on receptor expression.