Human eosinophils contain a number of granule proteins for which specific physiologic roles remain unclear. The combined ribonucleolytic and membrane disruptive properties of eosinophil-derived neurotoxin (EDN) and eosinophil cationic protein (ECP) suggest the possibility that eosinophils participate in host defense against enveloped single stranded RNA viruses. To investigate this hypothesis, a replication defective retrovirus encoding the reporter gene beta-galactosidase was preincubated with isolated human eosinophils, then used to transduce human erythroleukemia target cells (K-562). Histochemical staining for beta-galactosidase activity was used to determine the number of transduced target cells. Co-incubation of virus with eosinophils prior to infecting K-562 target cells resulted in a marked decrease in transduction efficiency corresponding to a 8-12 fold dilution of viral stock (p<.01), an effect that was directly proportional to the concentration of eosinophils used. Furthermore, when the virus was preincubated in the presence of eosinophils and ribonuclease inhibitor, transduction efficiency reverted to levels seen under the control conditions. Pre-incubation of virus with neutrophils, monocytes, or K562 cells served as negative controls. Other enveloped single stranded RNA viruses such as members of the family Paramyxoviridae (eg. respiratory syncytial virus (RSV), parainfluenza) may also be susceptible to this mechanism of host defense. Using a modified shell vial assay, we found that preincubation of RSV with eosinophils decreases the infectivity of RSV in a ribonuclease dependent fashion corresponding to a 6-10 fold dilution of viral stock (p<.01). These results demonstrate that eosinophils interfere with RSV transduction of human target cells via a ribonuclease-dependent mechanism, and suggest the possibility that viral particles can be destroyed by the actions of secreted eosinophil granule proteins.