Vesicular stomatitis virus (VSV) selectively assembled proteins from human cells into progeny virions. These proteins can be surface labeled before infection with 125I, and when purified virus was examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, only two or three bands of proteins (Mr around 100K) were seen. Antisera to these proteins were produced, using as immunizing antigen VSV tsO45 mutant, defective in assembly of G protein, which had been made at the nonpermissive temperature in the three human tumor cell lines, HeLa (cervical carcinoma), T47D (breast carcinoma), and HMB2 (melanoma). After absorption with wild-type VSV, each of the antisera displayed a different pattern of reactivity; at least three antigenic specificities were detected. Two of them, corresponding to antigens selected by VSV from HeLa and T47D, were to some extent related and they showed an association mainly with epithelial cell-derived gynecological tumors, but they were absent in carcinomas of lung or of digestive tract. These (or related) antigens were expressed in a lower level in some normal tissues, mainly in ovaries. Antigen(s) assembled by VSV from the melanoma cell line was entirely different and appeared to be associated with cell growth. The grounds for selective assembly of these specific proteins by VSV are not clear; they either share with viral surface glycoproteins some physical or structural properties, which are critical for incorporation into the viral envelope, or conceivably they even may represent uncleaved precursor proteins coded by env genes of incomplete genomes of endogenous human retroviruses.