The means for analysing T-cell activities are very limited. Indeed, the numerical estimation of the changes which take place in human T lymphocytes had to rely solely on the observation of transformed cells (blasts). However, even if various mitogens appear selective for either T of B cells, we know that both types of lymphocytes are ultimately transformed into blasts. This is particularly true in studies conducted on mixtures of T and B cells where stimulated T cells can release substances which act on B cells. On the other hand, studies with purified populations of T or B cells do not express the cellular interactions that occur naturally between these two types of lymphocytes. For all these reasons, it seemed that the estimation of " quanta " of activity unique to T lymphocytes would contribute significantly to our knowledge of this line of cells. We describe a new method of detecting the in vitro stimulation of lymphocytes based on the appearance of cells having the property to cluster several layers of sheep red blood cells (SRBC) around themselves, forming multilayer rosettes (CFC). This formation is temperature-dependent and requires trypanblue-excluding, metabolically active blastoid cells. Non-separated cells as well as purified T cells stimulated with allogeneic leucocytes (MLR), specific antigens or polyclonal mitogens (PWM, ConA) gave rise to CFC. Such multilayer rosettes were not formed by separated B cells. In the MLR, CFC were detected 48 h after beginning of cultures and increased in number thereafter just like thymidine incorporation. The response to ConA and PWM was detected earlier and gave rise to two or three peaks, the first rise in the number of CFC coinciding with the peak of thymidine incorporation, but the maximal increase occurring two or three days later. Treatment of blastoid cells with a serum specific for T cells--but not with an anti-immunoglobulin serum--abolished their ability to form ordinary or multilayer rosettes. When separated, however, on a nylon wool column, CFC were more adherent than the bulk of T cells. It is suggested that CFC form a subpopulation of T cells with distinct characteristics, allowing a direct assessment of membrane changes which take place when T lymphocytes are activated in vitro.