MaTu is a quasi-viral agent presumably derived from a human mammary tumor. In some respects it resembles classical viruses and in some the "slow viruses," and in others it is different from both. Using monoclonal antibodies (Mabs), we showed that it is a two-component system. One part of the complex, MX, is exogenous; it is manifested by a protein, p58X, which is a cytoplasmic antigen and it reacts with some natural sera of man and of various animals. The other component, MN, is endogenous to human cells. This is manifested by a twin protein(s), p54/58N, localized on the cell surface and in the nucleus. This protein is absent in rapidly growing, sparse cultures of HeLa, but it is inducible either by keeping the cells in dense cultures or, more efficiently, by infecting them with MX. Both inducing factors are synergistic. Only p54/58N is associated with virions of vesicular stomatitis virus (VSV), reproduced in MaTu-infected HeLa. This suggests that MN is responsible for complementation of VSV mutants and for the formation of the VSV (MaTu) pseudotype. Both p54/58N peptides are glycosylated and they form oligomers linked by disulfidic bonds; p58X is not glycosylated and it does not form S-S-linked oligomers.