Previous studies in Durban have shown that serological investigations, in combination with iso-enzyme electrophoresis, are invaluable for monitoring the endemicity of pathogenic strains of Entamoeba histolytica. We therefore proposed that antibody profiles could be used to detect epidemic situations. An outbreak of amoebiasis in the normally non-endemic Philippi area near Cape Town provided an opportunity for testing this hypothesis. Seven of 9 patients presenting at a district hospital with invasive amoebiasis originated from a single farm in Philippi. Iso-enzyme electrophoresis and serological investigations were used to monitor the endemicity of amoebiasis on 16 of the 49 farms in this district. In an attempt to contain disease transmission all inhabitants on farms from which patients came (including those where cyst-passers were identified) and all seropositive subjects were treated. The antibody profiles proved invaluable for confirming that the farm from which the hospitalised patients originated was the central focus of the outbreak, and also identified subjects infected with pathogenic zymodemes of E. histolytica on the adjacent 4 farms. On all 5 of these farms, 62.5-100% of seropositive subjects were strongly positive. In contrast weak to negative serological responses occurred on the remaining 11 farms. In addition the success of treatment was indicated by a notable drop in strong seropositive responses on the affected 5 farms to 11.5% within 9 months. The infection pathways implied that the pathogenic strain of E. histolytica was introduced into this non-endemic area by a foreigner from an endemic area; this suggests that the pathogenicity of E. histolytica is an immutable stable feature.