We detail in this report the characterization of a human B-lymphoblastoid cell line, .221, that does not express endogenous HLA-A, HLA-B, or HLA-C class I Ag due to gamma-ray-induced mutations in the HLA complex. Mutant .221 is characterized by: 1) complete absence of HLA-A,-B,-C mRNA transcripts and alpha-chains, and 2) intracellular expression of two non-A,-B,-C class I alpha-chains with an abundance less than or equal to 1% of normal HLA-A,-B,-C expression on similar cells. However, transferred HLA-A, HLA-B, and HLA-C genomic genes are expressed as cell surface Ag in amounts similar to expression of the same endogenous genes in human B-lymphoblastoid cells. The amount of class I transcript produced from transferred class I genes is roughly proportional to the number of gene copies but, in every case studied, post-transcriptional processes limited cell surface Ag expressions to amounts approximately normal for the cell type. The ability of mutant .221 to express quantitatively normal amounts of transferred class I genes suggests that: 1) it can serve as a recipient for, and then express, any cloned HLA-A,-B, or -C gene that would normally be expressible in human B-lymphoblastoid cells; 2) the absence of a background of HLA-A,-B,-C Ag permits its use for studying the expression of normal non-A,-B,-C class I genes and of class I genes that have mutations; 3) mutant .221 can be used to create human cells that express on their surfaces just one defined class I Ag encoded by a transferred class I gene.