CD4 is the predominant cell membrane protein that binds human immunodeficiency virus type 1 (HIV-1) gp120 and facilitates HIV-1 infection, but other membrane-associated molecules may be involved in determining HIV-1 cellular infection. Our prior work had suggested that CD44, the transmembrane receptor for hyaluronan, might play a role in the infection of mononuclear phagocytes with HIV-1. In the present work, we have used cells of the CD4-positive, CD44-negative human T-lymphoblast cell line Jurkat to study the role of CD44 in HIV-1 infection and tropism. Cells were transfected with cDNA for the standard (S, or hematopoietic) CD44 isoform CD44S or the epithelial isoform CD44E. The resultant lines expressed appropriate CD44S or CD44E mRNA and protein. While the parent Jurkat cells, those transfected with vector alone, and those transfected with CD44E could be productively infected with only the lymphocytotropic strain HIV-1-LAI, cells transfected with CD44S were rendered susceptible to productive infection with the monocytotropic strains HIV-1-BaL and HIV-1-ADA. Also, CD44S-transfected cells displayed higher levels of infection with HIV-1-LAI than did the other transfected Jurkat cells. The transfected cell line cells all had comparable growth rates and expressed similar levels of the membrane antigens CD4, CD7, major histocompatibility complex (MHC) class I, MHC class II, and CD11a, while levels of CD3 were slightly higher in cells transfected with vector alone and in one of the clones transfected with CD44S. Hyaluronan binding was increased in cells transfected with either CD44S or CD44E. Mouse NIH 3T3 fibroblasts transfected with human CD4, human CD44S, or both human CD4 and CD44S displayed the appropriate antigens, but they could not be productively infected with lymphocytotropic or monocytotropic strains of HIV-1. The results indicate that in human leukocytes, CD44S is an important determinant of HIV-1 productive infection and may be involved in viral cellular tropism.