Infestations of humans with the parasitic nematode T. canis are common in both developing and industrialized countries. Most infestations induce a clinically inapparent course of infection, however, severe clinical manifestations, i.e. visceral larva migrans (VLM) or ocular larva migrans (OLM) syndromes are observed. To find an explanation for the different courses of toxocarosis we examined several serological parameters: the expression of (i) specific IgE (Immunoblot, IB), (ii) specific IgG subclasses (IgG1-4, ELISA and the formation of (iii) IgE/anti-IgE immune complexes. Serum samples were obtained from persons with symptomatic (VLM, OLM) and asymptomatic course (AS) of the infestation. As antigen, T. canis excretory/secretory (TES) antigen from L3 larvae was used. Reactivity of IgE against SDS-PAGE separated TES antigens was marginally higher in toxocarosis patients (35%) than in asymptomatics (24%), but without statistical significance. TES-specific IgG (1-4), predominant subclass in all three groups was IgG1, followed by IgG2, IgG4 and IgG3. Subclass IgG1, 2, 4 showed significant differences between patients with VLM associated symptoms and asymptomatic persons (P < 0.001) but not between patients with OLM associated symptoms and asymptomatics. Significantly elevated levels of IgE/anti-IgE immune complexes were detected in sera of patients with symptomatic course of the disease, both VLM and OLM (P < 0.001). Whereas specific IgG may act via antibody dependent cell-mediated cytotoxicity mechanisms, IgE/anti-IgE immune complexes might possibly participate in VLM and OLM by inducing type III hypersensitivity.