Failure to detect human T-cell leukemia virus-related sequences in multiple sclerosis blood. We tested 11 patients with multiple sclerosis for the presence of human T-cell leukemia virus type I (HTLV-I)- or type II (HTLV-II)-related sequences. DNA from blood mononuclear cells was analyzed by the polymerase chain reaction utilizing three different oligonucleotide primer pairs. Two of these primer pairs detect sequences shared between HTLV-I and HTLV-II in either p24, gag protein, or in p21, env transmembrane protein. The third primer pair was synthesized based on regions in the pol gene where amino acid sequences are conserved between HTLV-I, HTLV-II, and the related bovine leukemia virus. The multiple sclerosis samples were consistently negative while appropriate control samples were positive. We conclude that viruses related to HTLV-I, HTLV-II, or bovine leukemia virus are not present in the blood of patients with multiple sclerosis and, therefore, that HTLV-bovine leukemia virus-related viruses are not likely to be involved in the pathogenesis of multiple sclerosis. 