Fc-rosetting and nonrosetting T cells are killer cells.

A fraction of T cells activated to H-2 determinants in vivo (ATO) express Fc receptors (FcR) on the cell surface. Those FcR can be detected by antibody coated red blood cells (EA rosettes). Nonrosetting and EA rosetting ATC could be obtained in highly enriched form (more than 99% pure) by separation by velocity sedimentation at unit gravity. Separately tested, both fractions of ATC showed killer cell activity against specific target cells. Possible explanations why only one population of T killer cells expressed FcR are discussed.