Children with leukocyte adherence deficiency (LAD), or leukocyte cell adhesion molecule deficiency, experience recurrent, life-threatening bacterial infections related to severe deficiency in surface expression of the leukocyte integrin molecules. The leukocyte integrins consist of a common CD18 (beta) subunit and individual, noncovalently associated alpha subunits designated CD11a, CD11b, and CD11c. Defects in the CD18 subunit prevent surface expression of the CD11/CD18 complexes in children with this disease. We investigated the molecular basis of the disease in a child with the severe deficiency form of LAD and identified two molecular defects in the CD18 subunit. The first defect is a single-base pair C----T transposition resulting in an amino acid substitution of a leucine for a proline at amino acid 178. This amino acid substitution is located in a region that is highly conserved among the integrin beta subunits and where two previous defects have been located in LAD. The second mutation involves a deletion of 220 base pairs in the cDNA coding for a portion of the extracellular domain and results in a frameshift into a premature stop codon. The deleted region corresponds to a single exon in the CD18 gene. Identification of these two molecular defects in a single child with this disease indicates the compound heterozygous nature of the disorder in this child and identifies regions of the CD18 subunit that may be important for CD11/CD18 heterodimer formation and surface expression.