We report a novel human thymocyte differentiation antigen ICT-1 with a molecular weight of 49 kDa that is noncovalently associated with another 12-kDa protein. The ICT-1 antigen is expressed in 50-70% of total thymocytes, but not in resting or PHA-activated peripheral blood T-cells and bone marrow cells. The thymocytes expressing ICT-1 antigen appear after the 18th week of gestation during fetal development. Since the distribution pattern of the ICT-1 antigen within thymus partly overlaps with that of the CD1 antigens, we investigated whether ICT-1 was one of the CD1 antigen family. However, the failure of anti-ICT-1 antibody to react with mouse L cells transfected with cDNA of CD1a, -b, and -c and the different histologic distribution patterns from that of CD1d strongly suggest that the anti-ICT-1 antibody recognizes an antigen distinct from CD1. Furthermore, ICT-1 is also expressed in human neuroglial cells such as oligodendroglioma, glioblastoma multiforme, Ewing’s sarcoma, and cerebellar astrocyte. Hence we believe that the ICT-1 antigen may be a novel thymus-leukemia (TL) antigen or a nonclassical MHC class I antigen.