The monoclonal antibody (MAB) 60.3 reacts with the beta chain (CD18) of lymphocyte function associated antigen (LFA-1), encoded by a gene on chromosome 21. We have used 60.3 to measure the expression of CD18 on lymphoblastoid cell-lines derived from persons with Trisomy 21 (Down syndrome) by quantitative fluorescence flow cytometric analysis. This showed that CD18 expression is increased on Trisomy 21 compared with normal cells, whereas neither HLA-class I, nor transferrin receptor expression are significantly affected. Similar results were obtained with the CD18 MAB MHM 23, and with the CD11a MAB MHM 24 (LFA-1 alpha). These results suggest that the increase in CD18 expression by Trisomy 21 cells is due to gene dosage, and could influence the immune status of persons with Down syndrome.