The expression of Fc gamma receptor III (Fc gamma RIII) on a human eosinophilic leukemia cell line, EoL-1, was examined and compared with its expression on normal blood eosinophils. Surface Fc gamma RIII expression on EoL-1 cells could be induced in vitro with a combination of dibutyryl cAMP (dbcAMP) and gamma-interferon (IFN-gamma), but not with IFN-gamma or dbcAMP alone. Pretreatment of EoL-1 cells with dbcAMP induced EoL-1 cells to express Fc gamma RIII when stimulated with IFN-gamma, but EoL-1 cells pretreated with IFN-gamma and then stimulated with dbcAMP failed to express Fc gamma RIII. Cyclic AMP was shown to play a role in the effect of dbcAMP. Both the treatment with phosphatidyl-inositol-specific phospholipase C (PI-PLC) and the restriction enzyme digestion of Fc gamma RIII cDNA showed that the Fc gamma RIII on EoL-1 cells was a phosphatidylinositol-linked form. On the other hand, freshly isolated blood eosinophils constitutively expressed few, if any, Fc gamma RIII, and IFN-gamma induced Fc gamma RIII expression on them in vitro. Dibutyryl cAMP did not induce Fc gamma RIII expression and even suppressed the IFN-gamma-induced Fc gamma RIII expression on normal eosinophils. The EoL-1 cell line appears to be a useful in vitro model for the expression and function of the phosphatidylinositol-linked form of Fc gamma RIII on eosinophils.