Monoclonal antibodies (mAbs) W6/32, HC10, and 4E were used to precipitate class I antigens from 21 selected individuals with at least one HLA-C "blank" allele. In 19 of these individuals, characteristic HLA-C banding patterns which could be precipitated by all three HLA class I mAbs were observed on one-dimensional isoelectric focusing gels--obviously the gene products of HLA-C "blank". At least four allelic HLA-C "blank" gene products with different isoelectric points could be discerned. All of them segregated with HLA-C "blank" haplotypes in informative families; two of them were associated with HLA-B51, one with HLA-B38, and one with HLA-B18. Reactivity of the HLA-C "blank" heavy chains with mAb W6/32 indicates that they are able to associate with beta-2 microglobulin, and hence are most probably expressed at the cell surface.