We found a type D retrovirus in a human lymphoblastoid cell line of B-cell lineage. Molecular cloning and nucleotide sequencing of the provirus genome revealed that this virus was closely related to squirrel monkey retrovirus (SMRV), and we designated this virus as SMRV-H. To investigate the relationship between these retroviruses, SMRV-H was purified from the virus-producing cells, and its biochemical properties were characterized. The cell-adhesive virus particles were successfully separated from the cell by a brief trypsin treatment and purified by velocity sedimentation. The purification of the virus was confirmed by electron microscopy. Major gag protein of the virus is phosphorylated, and has a molecular weight of 34 kDa. The virion-associated reverse transcriptase prefers Mg2+ to Mn2+. These properties of SMRV-H were almost the same as those of SMRV.