To elucidate the function of nef, we constructed infectious chimeric clones between human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) of macaques, by deleting a part of the nef sequence from the HIV-1 genome and inserting the entire nef gene from the pathogenic molecular clone, SIVmac239. We have named this construct HSIVnef. While there was no difference in the replication kinetics of HSIVnef+ and HSIVnef- in transformed cell lines, there was a profound difference in the replication of these virus in primary cells. These data indicated that the SIV nef gene could complement the functions of the HIV-1 nef gene in the context of the HIV-1 genome, providing further evidence that results observed in the macaque model are relevant to HIV infections in man. HSIVnef is a useful model for the study of the nef gene.