Dot-blot analysis of whole-cell suspensions of meningococci showed that 81% of B:15:P1.16 strains from patients reacted with a monoclonal antibody (MAb) against subtype P1.7. The remaining strains, which did not react on dot-blots or in ELISA, demonstrated the P1.7 subtype epitope on immunoblots after denaturation of the cells with sodium dodecyl sulphate. The monomeric class 1 proteins of the two P1.16 subtype variants had slightly different mol. wts, but bound the P1.7 antibody equally well. These results were explained by a deletion of three codons in the gene encoding the first variable region of the P1.16 class 1 protein. The deletion accounted for the non-exposure of the P1.7 epitope on native cells. Other patient strains, with subtypes P1.3, P1.9 or without any known subtype, also showed a binding site for the P1.7 MAb, which became available only after denaturation. Demonstration of inaccessible epitopes may have consequences for subtype designations and vaccine development.