We determined the susceptibility of monocytic cell lines to infection with viral strains derived from two infectious clones of simian immunodeficiency virus isolated from a mandrill. One of the strains, which replicates poorly in T cell lines, was found to grow more rapidly than the other in these cells. The viral determinant for this property was genetically mapped within the env, gene encoding a surface protein. Six amino acid substitutions identified appeared to be located outside of the domains corresponding to human immunodeficiency virus type 1 env functional domains such as the CD4-binding and V3 loop regions.