We examined the capacity of interleukin-4 (IL) production from lymphocytes and basophils, isolated from the peripheral blood of allergic patients sensitive to house dust mite, after stimulation with mite extract. IL-4 production was measured by a sensitive bioassay based on coculture with CT.h4S (a human IL-4-responsive cell line). Lymphocytes and basophils from patients with elevated serum IgE specific to mite allergen [radioallergosorbent test (RAST) score > 3] could produce detectable levels of IL-4 in response to mite extract, whereas those from patients with a RAST score of less than 2 or normal volunteers could not. The sensitivity of basophils to mite extract was high, so that a lower concentration of mite extract (1-10 ng/ml) could induce maximal IL-4 production. On the other hand, a higher concentration (10 micrograms/ml) was required for maximal IL-4 production from the lymphocytes. These findings demonstrate that allergen-specific IL-4-producing cells, lymphocytes and basophils, are generated in vivo in allergic patients and also that there exist characteristic differences between lymphocytes and basophils related to the in vivo source of IL-4.