ABSTRACT.Monoclonal antibodies that react with the circumsporozoite protein of the avian malariaPlasmodium gallinaceumsporozoites also reacted with circumsporozoite protein of the rodent malariaPlasmodium berghei.Two types of reactivity were identified: 1) two monoclonal antibodies reacted withP. bergheisporozoite protein by enzyme‐linked immunosorbent assay, Western blot and indirect immunofluorescence antibody, 2) six other monoclonal antibodies reacted withP. bergheisporozoites by ELISA and Western blot only. We studied whether these differences could be explained by reactivity in enzyme‐linked immunosorbent assay with differentP. bergheicircumsporozoite peptides. Although allP. gallinaceummonoclonal antibodies reacted with theP. bergheirepeats, the first group reacted with a conserved peptide sequence, N1, whereas the second group did not. These results suggest that circumsporozoite proteins fromP. gallinaceumandP. bergheishare common epitopes. the biological significance of our finding is not yet clear. Indeed, the cross‐reactive monoclonal antibodies giving a positive indirect immunofluorescence antibody with theP. bergheisporozoites only caused a borderline effect on the livingP. bergheiparasites in vitro as measured by inhibition of sporozoite infect