Four of 23 H-2 alloantisera screened for anti-TL activity contained such activity. One of these alloantisera, D-35, defined a new TL specificity, TL.5, and a new Tla allele, Tlad. TL.5 has all the characteristics of a TL antigen and has a different strain distribution than previously known ones. This new complexity at the Tla locus and the previous finding of other serologically defined genes in the Tla region indicate that this genetic region cannot be ignored in analyzing antisera produced in strains made congenic for the major histocompatibility complex.