Using the micropore filter technique of Boyden, the eosinophil chemotactic responses to an ECF-A tetrapeptide (Val-Gly-Ser-Glu) and histamine were studied in thirty-two patients with a peripheral blood eosinophilia. The eosinophilia was associated with extrinsic asthma (twelve patients), neoplasia (seven), helminth disease (five) and a miscellaneous group (eight) which included three patients with pulmonary eosinophilia, one of whom also had asthma. Both the tetrapeptide and histamine gave two types of dose–response. With Val-Gly-Ser-Glu this was either a single peak at 10-8, or two peaks at 10-4 and 10-7 mol/1, respectively. Histamine gave either a linear dose–response with maximal chemotaxis at the highest concentration or a peak response at 10-5 mol/1 with inhibition at higher doses. The two types of response given by the peptide were not related to the disease state and were reproducible when tested on more than one occasion. However, with histamine, linear dose–responses were observed in ten out of twelve patients with asthma, four out of five with helminth disease and two of the three with pulmonary eosinophilia. This was also reproducible when tested on subsequent occasions. Therefore, in these diseases, which are known to be associated with exogenous antigens and raised IgE levels, eosinophils from 80% of the patients studied (sixteen out of twenty) gave a linear rather than a ‘bell-shaped’ response to histamine. In contrast, only four of twelve patients(33%) with eosinophilia in association with other diseases (which included seven with neoplasia) gave this type of response. If this observation can be confirmed with larger numbers of patients it may be useful diagnostically in eosinophilia of unknown origin.