Preliminary studies in hematological patients have indicated that treatment with rhG-CSF reduces basophil releasability ex vivo. We examined this phenomenon further, in allergic patients. Ten patients with grass pollen rhinoconjunctivitis were given rhG-CSF (5 μg/kg/day s.c.) for 5 days, and examined before and after treatment. Basophil counts increased from 5 to 19×109/l (P<0.01). Total blood histamine increased from 80 to 160 μg/l (P<0.01), corresponding to a decrease in average basophil histamine content from 1.5 to 0.81 pg/cell (P<0.01). Isolated mononuclear cells showed a significantly decreased histamine release (HR) when stimulated with A23187 and grass. Whole blood experiments showed a similar decreased HR to grass and anti-IgE (P<0.01). However, we found an increase in total blood histamine. We conclude that treatment with rhG-CSF (1) increases the number of circulating blood basophils, (2) reduces the average histamine content per basophil, and (3) reduces the basophil releasability. These findings could be due to the mobilization of immature basophils from the bone marrow.