B cell epitopes of the measles virus fusion protein were mapped, by reacting sera from late convalescent donors with synthetic overlapping pentadecapeptides, segments covering the whole F protein sequence. Unselected individual sera recognized 7 to 20% of the total sequence. Cumulation of the binding patterns of 30 sera identified eight to 10 clusters of antibody-binding peptides spread over most of the sequence. The B cell epitopes included regions of transition between the more hydropathic (including the N-terminal end of the F1 and F2 protein) and hydrophilic sequences. When the regions of antibody binding were compared with the predicted secondary structure of the F protein, no detectable pattern became apparent. Exposed sequences as well as sequences hidden in the viral membrane or in the protein core of both the F1 and F2 polypeptides were recognized by the antibodies. The heterogeneity of the binding patterns was not merely dependent on the anti-measles virus titre. The importance of antibodies recognizing linear epitopes of the measles virus fusion protein for the immune protection is presently not known.