Fragments of the 5’ long terminal repeat (L TR) of the human endogenous retroviral element, HRES-I, were amplified. Single strand conformation analysis of these fragments in combination with sequencing revealed two polymorphic nucleotides, namely a HindIII and an Eco57I polymorphic site. Moreover, a number of differences from the previously published HRES-l L TR sequence were detected. The linkage pattern of the two polymorph isms suggested the existence of three allelic forms of HRES-l. The frequencies of the genotypes and alleles of HRES-I were determined in 158 individuals. The detection of HRES-I markers may be useful to study associations between this endogenous retrovirus and various diseases.