The uptake of the monoamine radiolabelled tracer 125I-meta-iodo-benzyl-guanidine (MIBG) was studied in vitro in platelets from 15 healthy volunteers and five allergic patients whose serum IgE concentration was 37 ± 33 and 650 ± 200 IU/ml, respectively. The MIBG uptake was determined by means of a gamma counter after incubation in a buffer. Binding of IgE to platelets significantly reduced MIBG uptake both in healthy subjects and in allergic patients. This reduction was significantly related to the action of IgE as it did not occur after binding of IgG. Moreover, MIBG uptake plotted versus increasing concentration of IgE (ranging from 10 to 3000 IU/ml) was fitted by a classical sigmoidal curve which was not significantly different between healthy subjects and allergic patients. Lastly, the effect of IgE on MIBG uptake was due to the binding of the sole IgE as it was not modified by subsequent addition of anti-IgE. It is concluded that the binding of the sole IgE to its specific receptor on platelets alters the transport of monoamines.