To clarify the nature of tumor-infiltrating lymphocytes (TILs), we investigated the possible clonality of the T cells in TlLs freshly isolated from human primary lung cancer tissues by assessing the rearrangement pattern of the T-cell receptor (TCR) gene jl locus using Southern blotting. First, in phenotypic analysis, TlLs represented different populations among corresponding peripheral blood lymphocytes (PBls) with an increased proportion of CD2O+ (6) cells as well as a decreased proportion of C D I6+ (natural killer) cells, and a variable CD4/ CD8 ratio. Considering the central role of T cells in immune responses, we analyzed TCR jl gene rearrangement patterns in T l b and corresponding PBLs from I 2 patients. In 10 of the I 2 cases, TlLs showed one or more TCR gene rearrangement bands with a predominance of the C 2 gene, in which 2 types of common rearranged band were o%served among the cases with different clinical profiles in terms of histological types and disease stage, with bands at about 9.5 kb in 7 and at I I .5 kb in 8 patients. On the other hand, predominant rearranged bands were hardly detected in corresponding PBLs except in 2 cases. From these results, we conclude that Tlls in lung cancer tissues frequently contain oligoclonal T-cell populations, which were probably sensitized by relatively common antigens at the tumor sites.