To investigate the role of IgE-bearing Langerhans' cells (LC) from atopic dermatitis (AD) patients in antigen presentation, IgE+LC and non-IgE bearing LC (IgE-LC) from AD patients were investigated for their antigen-presenting capacity and compared to antigen-presenting cells (APC) from peripheral blood. The T-cell response to Candida albicans, using IgE+LC from AD patients as APC, was in the same range as with IgE-LC. Also, the T-cell response to Candida with autologous APC from peripheral blood did not significantly differ between these groups. In contrast to this finding, the T-cell response to house dust allergen (HDA) was dependent on the type of APC used. If non-T cells from peripheral blood were used as APC, both AD patients and controls responded to HDA. However, when LC were used as APC, a T-cell response to HDA was only observed in the presence of IgE+LC. IgE-LC from AD patients or LC from normal controls were unable to present HDA. Preincubation of IgE+LC with anti-IgE or anti-kappa/lambda antibodies inhibited HDA-induced T-cell proliferation, whereas the response to Candida was not affected. These in vitro results, which demonstrate the necessity of cell-bound IgE on LC for the presentation of aero-allergens, strongly correlate with the in vivo presence of a positive delayed patch reaction to the same antigens. When the LC of a patient appeared to be IgE-, the in vitro proliferative response as well as, in most cases, the in vivo patch test reaction to the same antigens was negative. In conclusion, these experiments demonstrate that there are at least two different mechanisms by which LC capture antigens for antigen presentation. In one of them cell-bound IgE, as can be demonstrated on LC from AD patients, plays a crucial role. The binding and presentation of HDA by APC from peripheral blood can take place independently of cell-bound IgE. Candida can be presented by both types of APC in the absence of IgE.