A major portion of the nucleoprotein (amino acids 67 through 300) and the glycoprotein-2 of lymphocytic choriomeningitis (LCM) virus were synthesized by using recombinant technology and were injected together with SDS twice in portions of 5 micrograms into BALB/c mice. As evidenced by diminished replication of LCM challenge virus, both proteins induced antiviral immunity, which was comparable in extent with the immunity caused by infection with LCM vaccinia recombinant viruses. Primed LCM-viral CTLs could not be demonstrated in these mice by culturing splenocytes in the presence of LCM virus, and Abs appeared slowly and in low quantities; but, after injection of large infectious doses, CTLs appeared faster and in higher numbers than in mice not previously treated with viral proteins. Depletion of CD8+ cells, but not of CD4+ cells, by treatment of mice with mAb abolished the antiviral immunity, demonstrating that protection was mediated by CD8+ T lymphocytes. Absence of CD4+ T lymphocytes before and during the period of immunization did not measurably affect the animals' antiviral immune status, indicating that activation of the CD8+ T lymphocytes was not dependent on help by CD4+ cells.