Novel immunogenic compositions are provided involving viral particles composed at least in part of hybrid proteins of at least a portion of a particle forming protein and one or more polypeptides having at least one epitope of interest. Nucleic acid sequences are employed coding for the hybrid protein which are introduced into a host cell for expression, either by themselves or in combination with other DNA sequences coding for particle forming proteins. Expression of the DNA sequences results in formation of particles which may be isolated and used as immunogens for production of antibodies for diagnostics purposes, passive immunization, vaccination, or other uses. Saccharomyces carlsbergensis, 2150-2-3 (pDC103), was deposited on Sep. 7, 1984, at the ATCC and given ATCC Accession No. 20726. Also, Saccharomyces cerevisiae PO17 (pC1/1-MCS29) was deposited at the ATCC on Sep. 5, 1985, and given ATCC Accession No. 20770.