Heat-aggregated guinea pig gamma-globulin was shown to bind to the surface membrane of a subclass of guinea pig T lymphocytes. Cells of this subpopulation were identified as T lymphocytes because these cells did not stain for surface Ig (a B-cell marker) but did form spontaneous E-rosettes with rabbit erythrocytes (a T-cell marker). A strikingly high proportion of such aggregate-binding (Agg(+)), E-rosette-forming (E-rosette(+)), but surface Ig-negative (Ig(-)) cells were found in an inflammatory exudate. Thus purified peritoneal exudate lymphocytes (PELs) are known to consist of over 90% T cells, and 59% of these cells bound aggregates. 10% of these Agg+ Ig- E-rosette+ cells were found in draining lymph node cell populations and none in thymus cell populations. The high frequency amongst PELs suggested that these Aggregate+ Ig- E-rosette+ cells might be activated T cells as these are known to occur in high proportion in PEL populations. Confirmatory evidence for this postulate was provided by the striking increase (from 10% to 46%) of Ig- E-rosette+ cells that bound aggregates when lymph node cells were activated by antigen stimulation in vitro.