An immunoelectrotransfer blot assay (IETB) using excretory–secretory products of muscle larvae of Trichinella spiralis (ML-ESP) and the avidin–biotin system was developed in order to characterize reactivity against ML-ESP in sera from patients with acute and chronic trichinellosis. A complete pattern of up to 13 bands was developed by sera from individuals with trichinellosis where doublets, triplets, or single bands were shown to have molecular weights of roughly 66, 55, 45, 36, 29, 24, and 14 kDa. The bands at ∼55, 36, 29, and 14 kDa proved specific for T. spiralis. The band at ∼55 kDa was present in all trichinellosis sera, whereas the ∼14-kDa band was present in only a small percentage of sera. The development of ∼36- and 29-kDa bands suggests a modulation of the reactivity against ML-ESP over time. IETB proved more sensitive for the population of chronic trichinellosis under study than a conventional diagnostic enzyme-linked immunosorbent assay, allowing negative or borderline serum samples to be determined. Thus, this technique, when applied for human trichinellosis surveillance, should provide a useful tool in endemic areas.