Under appropriate culture conditions, EB virus infection of lymphocytes from seropositive donors leads to regression of transformation, and this was shown previously to be due to activation in a secondary immune response to T lymphocytes inhibitory for the autologous lymphoblastoid cell line. Regression can be quantified by determining the number of cells required for its expression. To investigate the development of memory T cells with EB-virus specificity in the primary infection, a comparison was made of the capacity for regression of lymphocytes from 16 cases of infectious mononucleosis (IM) and 13 normal donors. With 9 normal seropositive donors a mean lymphocyte concentration of 4.6 X 10(5)/ml was required to achieve 50% regression. In contrast, with 8 cases of IM tested within 1 week of onset, a much higher mean lymphocyte concentration (3.7 X 10(6)/ml) was necessary. Six of these IM cases, and another not tested in the first week, were tested on several occasions between 5 and 23 weeks after onset, and showed a slight reduction in the mean cell concentration required for regression (1.5 X 10(6)/ml). Six additional were tested 23--83 weeks after onset by which time the cell concentration required for 50% regression (mean = 4.5 X 10(5)/ml) had reached the level shown by normal seropositive donors. Regression did not occur with lymphocytes from seronegative donors, even at the highest cell concentration. Recombination cultures of T-cell-depleted and T-cell-enriched lymphocyte populations from 3 IM cases in ratios of 1:7 to 7:1 showed that the failure of regression in acute IM was not due simply to lack of sufficient numbers of T cells. The results indicate that EB-virus-specific memory T-cell activity as detected by the regression test is absent in the acute phase of IM, becomes evident at low levels 5--23 weeks after onset, and reaches a maximum after about 6 months.