The class 3 protein (PorB) is an important component of the meningococcal outer membrane. The structural gene (porB) encoding the class 3 protein has been cloned using primers suitable for the amplification of the corresponding chromosomal fragment by the polymerase chain reaction (PCR). The complete nucleotide sequence was determined and predicts a mature protein of 310 amino acids, preceded by a signal peptide of 19 residues. The predicted protein sequence of the class 3 protein exhibits essential structural homology to the gonococcal porin PIA. The class 3 protein encoding gene was expressed in Escherichia coli under the control of an inducible promoter.