The structural proteins of an arenavirus pathogen, Machupo virus, were compared to the structural proteins of two previously characterized non-pathogenic arenaviruses, Pichinde and Tacaribe, in SDS-polyacrylamide gels. Similarities in mol. wt. of the major structural proteins from both pathogenic and non-pathogenic viruses were apparent; however, some differences in the number of glycosylation properties of minor proteins were observed. Machupo virions contain two major protein species. The most prominent is a non-glycosylated protein with a mol. wt. of 68000, while the other was glycosylated protein with a mol. wt. of 41000. Minor amounts of other proteins (mol. wt. 84000, 74000, 50000 and 15000) and a glycolipid were also observed.