A new human T cell subset defined by antineuronal ganglioside mAb 3G5 increases linearly with advancing age in man. The percentage of circulating 3G5+ T cells in 21 normal individuals, quantitated by cytofluorograph analysis, increases linearly from age 7 (23-30%) to age 84 (58%) (r = 0.85, p less than 0.001). The antigen on T cells has the biochemical properties of a ganglioside that migrates between GM1 and GM2 ganglioside markers on TLC. The 3G5 subset represents the first T cell subset that reflects aging in man.