The yeast retrotransposon, Ty, encodes a set of proteins that are assembled into virus-like particles, Ty-VLPs (refs 1, 2). These proteins include Ty-VLP structural proteins, a protease that mediates cleavage of primary translation products and a reverse transcriptase. The major structural components of Ty-VLPs are proteolytic products of the primary translation product, p1 (ref. 3). We have recently shown that protein p1 alone can form Ty-VLPs (ref. 3). Here we demonstrate that p1 fusion proteins, comprising most of p1 and part of human immunodeficiency virus (HIV) protein gp120, form hybrid HIV:Ty-VLPs. These hybrid particles provide a rapid means of preparing and evaluating HIV antigens for a variety of immunological purposes.