Eosinophils synthesize and release a number of cytokines and chemokines, including RANTES, a potent chemoattractant particularly for memory T cells and eosinophils. Long-term (>12 h) incubation with interferon-gamma (IFN-gamma) has been shown to activate eosinophils and induce expression of membrane receptors. We hypothesized that IFN-gamma mobilizes intracellular RANTES in eosinophils in advance of mediator release. Highly purified peripheral blood eosinophils were obtained from asthmatics and stimulated with IFN-gamma at 500 U/ml for time course analysis up to 2h. By specific ELISA, RANTES was detected in supernatants (80+/-15 pg per 2x10(6) cells) following 120min of stimulation. Immunoreactive RANTES in resting cells (5x10(7) eosinophils) was detected in two intracellular compartments in studies of subcellular fractionation by density gradient centrifugation. After 10 min IFN-gamma stimulation, RANTES immunoreactivity was confined to crystalloid granules. RANTES was redistributed from secretory granules to light-membrane fractions after 60 min of IFN-gamma incubation. Our data suggest that rapid mobilization and release of RANTES occurs from stimulated eosinophils. These findings may have important implications for the role of IFN-gamma in activating human eosinophils, particularly in severe chronic asthma or viral exacerbation of asthmatic inflammation, where this cytokine may play a role.