We present a PCR-based Mse I restriction fragment length polymorphism (RFLP) in the promotor of the human CTLA4 gene at position -318 relative to the ATG start codon. In a random caucasian population including 239 individuals the allele frequency of the polymorphism was 13.4%, with a calculated heterozygosity rate of 23.2% and an observed one of 25.1%. CTLA4 and CD28 are two genes in close proximity on the long arm of chromosome 2. Both of them influence T-cell activity in an antigen-independent way. They represent candidate genes for several immunological disorders. Recently association between the CTLA4 gene and Graves' disease has been shown. The Mse I RFLP in the CTLA4 promotor might serve as a useful tool for further association studies.