Previous authors reported that in-vitro incubation with doxycycline reduced the number and length of filopods projecting from the surface of neutrophils. In the present study, control neutrophils were harvested from 11 probands at time zero. The probands were then given an oral single-dose of 400 mg doxycycline monohydrate. Test cells were harvested 24 h later. Control cells and test cells were compared by means of quantitative morphometry applied to electron microscopy. No effect of doxycycline treatment could be demonstrated on cell area, cell volume and cell surface density. The results suggested that there is no effect of conventional doxycycline treatment on neutrophil filopods.