This study focused its attention on four medicinal plants (Carapa guianensis, Elephantopus scaber, Piper umbellatum, Stachytarpheta cayenensis) used by Caboclo communities on Marajó, the main island of the Amazon delta. In the field, interviews were conducted with Caboclos and the medicinal usages and preparation procedures of the four plants were recorded. In the laboratory, the plant extracts were subjected to bioassays and their crude chemical composition was established. All four plants showed significant bioactivity and the chemical tests confirmed the presence of bioactive compounds. In addition, the results of both the field and laboratory studies corresponded well with those of a literature search. The ethnopharmacological significance of the four plants is discussed.