L ymphocyte subsets were assessed in patients with measles using the OKT range of monoclonal antibodies. A significant decrease in cells reacting with the OKT3 and OKT4 monoclonal antibodies was observed. When the tests were repeated 3 wk after the acute infection, significant recovery of these subsets was observed. The abnormality in lymphocyte subsets could be reproduced by treating normal lymphocytes with measles virus in vitro. When lymphocytes from patients with measles or when normal cells infected with measles virus in vitro were treated with either levamisole or L-ascorbic acid for 15 min and then retested with the OKT antisera, restoration of the previously depleted OKT3+ and OKT4+ cell population was observed. Ascorbic-acid treatment also, to a certain extent, reversed the inability of measles mononuclear cells to produce lymphocyte mitogenic factor (helper factor for B cells) after pokeweed mitogen activation. This abnormality, however, could not be reversed by in vitro treatment with levamisole. Measles patients treated with L-ascorbic acid demonstrated no accelerated recovery in either their lymphocyte subset profile or their ability to produce lymphocyte mitogenic factor. Although the cause of the depressed OKT3+ and OKT4+ lymphocyte subpopulations in patients with measles is not clear, the results suggest that the effect is not due to an aberration of protein synthesis, but rather to a blocking or steric change produced by the virus on the cell membrane. It is likely that both ascorbic acid and levamisole have the ability to reverse this effect by virtue of their antioxidant properties.