AbstractThree alloreactive T cell clones are described which reveal a new lymphocytestimulating determinant (Lsd) controlled by a gene on chromosome 1 of the mouse but different from and possibly centromeric to Mls. Two clones were originally isolated as anti‐H‐2b‐specific and showed and retained cross‐reactivity to Lsd (clone OD3, BALB/c anti‐C57BL/6; clone KB37, B10.BR anti‐C57BL/10). The third clone (BD7, C57BL/6 anti‐CBA/J) was probably originally directed against Lsd. So far, Lsd is only characterized by stimulation of T cell proliferation, and its physiological function is