Two ELISA assays were developed to test the reactivity of soluble sHLAs with anti-HLA class I mAbs of IgG or IgM isotype. A panel of 40 different alleles of sHLA antigens was produced using 57 lymphoblastoid B-cell lines, which had been generated from class-I-phenotyped PBLs. Using 14 mAbs, the expression of 13 different sHLA antigens (sHLA-A2, -A3, -A11, -A24, -A29, -B7, -B8, -B13, -B14, -B27, -B44, -B57, and -B58) by 43 different cell lines was confirmed. In addition, the expected absence of these alleles from the culture supernatant of 11 cell lines was confirmed. Cross-reactivities of mAbs observed by microlymphocytotoxicity assays were also detected by ELISA. The results of this extensive analysis confirmed previous results demonstrating that sHLA class I typing by ELISA correlated with HLA class I cell typing by microlymphocytotoxicity [1-3]. Furthermore, additional information about the fine specificities of two mAbs was obtained. An anti-B27/44 IgM mAb appeared to react only with sHLA-B44 but not with sHLA-B27; mAb CR11-351, previously reported to react with HLA-A2, 28, bound also to sHLA-A1, -3, -11, and -A24.