A variety of lymphoid cell populations were examined in terms of their ability to replicate vesicular stomatitis virus (VSV), a lytic, RNA-containing virus maturing at the cell surface. The number of cells capable of producing VSV was estimated in terms of infectious centers by the virus plaque assay (VPA), and morphologically by electron microscopy (EM). The lymphoid cells examined in this study included: (a) lymph node cells from delayed hypersensitive guinea pigs stimulated by specific antigen, (b) mouse spleen cells activated by selective bone marrow-derived (B) cell and thymus derived (T) cell mitogens, and (c) cells of human and murine continuous lymphoblastoid or lymphoma lines. In unstimulated cultures of guinea pig lymph node cells there is a background of approximately 1 in 1,000 cells which produces VSV; in purified protein derivative (PPD)-stimulated cultures the number of cells producing virus was 1.6% in the VPA and 1.9% by EM. These cells were large lymphocytes with some morphological features of transformed lymphocytes but were not typical blast cells. A few macrophages were associated with virus in both stimulated and control cultures. These observations indicate that (a) cells responsive to antigens, as detected by a marker virus, were lymphocytes; (b) cells other than lymphocytes (macrophages) were capable of replicating VSV even without antigenic stimulation; and (c) the correlation of results obtained by VPA and morphologic examination was usually quite good. Of the total number of mouse spleen cells stimulated with concanavalin (Con A), a T cell mitogen, 4.5 (EM)-5.7% (VPA) were associated with VSV. These were characteristic transformed lymphocytes, similar to phytohemagglutinin (PHA)-stimulated human lymphocytes. In contrast Escherichia coli lipopolysaccharide (LPS)-treated mouse spleen cultures contained lower numbers of virus plaque-forming cells. The majority of such cells associated with virus displayed extensive rough endoplasmic reticulum. Two cultured murine lymphomas containing lymphocytes with the theta surface marker (L5178Y and EL-4) showed a 15-100-fold higher incidence of virus-producing cells than leukemias (L1210 and C57Bl/6) which did not carry this marker. Similarly, the L2C guinea pig leukemia, a known B cell leukemia, yielded a low percent of virus plaque-forming cells (<2%). However, MOPC-104, a plasma cell tumor presumed to be of B cell origin, was found to be an efficient virus producer. There was a wide variation in the efficiency of VSV replication among human lymphoblastoid lines. One line, Wil-2, produced 80% infectious centers after 24 h of exposure to VSV, and all cells were associated with virus at the EM level. The relationship between the virus-producing cells and different lymphocyte subpopulations as well as the efficiency of the two assays for studying virus-producing lymphocytes is discussed.