Lymphocyte function associated antigen 1 (LFA-1) is a leukocyte cell adhesion protein. We have studied a novel human immunodeficiency disease in which LFA-1 and two other proteins which share the same beta subunit are lacking from the surface of leukocytes. The basis of the inherited defect in cell surface expression of both the alpha and beta subunits of LFA-1 was determined by somatic cell fusion of patient or normal human cells with an LFA-1+ mouse T cell line. Human LFA-1 alpha and beta subunits from normal cells could associate with mouse LFA-1 subunits to form interspecies hybrid alpha beta complexes. Surface expression of the alpha but not the beta subunit of patient cells was rescued by the formation of interspecies complexes. The findings show that the LFA-1 alpha subunit in genetically deficient cells is competent for surface expression in the presence of an appropriate beta subunit, and suggest that the genetic lesion affects the beta subunit. The human LFA-1 alpha and beta subunits were mapped to chromosomes 16 and 21, respectively. The genetic defect is inferred to be on chromosome 21.