The density distribution pattern of eosinophils over discontinuous isotonic Percoll gradients from the blood of normal, asymptomatic allergic and non-allergic asthmatic individuals was investigated. There was a completely identical distribution pattern between the investigated groups. Analysis of the expression of surface markers for complement receptors CR1 and CR3 and immunoglobulin G receptor on eosinophils derived from the density bands 1.080, 1.085 and 1.090 g/ml supported this finding since they did not reveal differences in expression between the bands within one group but also not between the three groups. Eosinophils of the various density bands were further purified and stimulated in vitro to produce leukotriene C4 (LTC4) by the calcium ionophore A23187 or serum treated zymosan. Equal amounts of LTC4 were synthesized by the eosinophils of the various density bands within one group. However, it appeared that the eosinophils of all density bands of allergic and non-allergic asthmatics synthesized significantly more LTC4 than the eosinophils from normal individuals (five- to tenfold). Probably this indicates in vivo priming of the eosinophils in asthmatic individuals which is not reflected by a change in density. Control experiments, dealing with possible artifacts due to the isolation procedure or the patient selection, to find differences in distribution patterns over discontinuous Percoll density gradients of the eosinophils of asthmatic compared to normal individuals failed to show such a difference. Therefore, the density distribution pattern of eosinophils over these gradients does not reflect cell activation, whereas LTC4 formation clearly does. This could mean that LTC4 formation is a more sensitive parameter for cell activation than density distribution or cell surface marker expression.