In this report seven different parameters were employed to investigate the spleen and lymph node cells from mice at the early and the late state of immunity to the lymphocytic choriomeningitis (LCM) virus. Distinct differences were observed. Morphological studies revealed a different size distribution of the cells in the preparations from the early and the late state of immunity. The cell mixtures of early immune cells contained many more large and blast-like lymphoid cells than the other. Where the cell function was concerned, the cytotoxic activity against LCM virus-infected target cells was almost entirely a function of the early immune cells, and our data strongly indicate that enhancement does not play any role for the disappearance by time of this cell activity. The antiviral effect after transfer to acutely infected animals was also predominantly a function of the early immune cells and the same was the case concerning the ability to protect against a lethal acute infection. However, the early immune cells were almost inactive after transfer to chronically infected virus carriers, whereas transplants of late immune cells to such mice had a very strong antiviral effect. The resistance to X irradiation also varied. Even high X-ray doses could not destroy the function of early immune cells, whereas the function of the late immune cells was readily impaired by X-ray treatment. The early and the late immune cells have one thing in common-both are susceptible to treatment with anti-theta serum. Because of the differences observed between the early and the late immune cells, it is concluded that they belong to different cell populations. However, because of the common susceptibility to anti-theta serum, probably both populations are T-cell lymphocytes. The implications of the results and the role of the different cells in the combat of the viral infection are discussed.