Fig. l A, B. Characteristics of known MHC-specific rnAbs and of the new SLA class I-specific mAbs. C'-mcdiated cytotoxicity assays were performed on pig PBMCs as described by Pescovitz and co-workers (1984), setting lysis as 100% for cells incubated with C'after PT85 (Davis et al. 1987), and lysis with C" alone as 0% lysis. Transfected mouse fibroblast lines, designated 0 [line 81 transfected with thymidine kinase (tk) DNA], PD1 (line 94 with PD1 and tk DNA), and PD14 (line 106 with PD14 and tk DNA) were gifts from D. Singer (NIH, Bethesda, Maryland; Singer et al. 1982, 1988). The antigen M r was determined by irnmunoprecipitation essentially as described by Pescovitz and co-workers (1984). miniature swine. Both mAbs were reactive with more than 90% ofddPBMCs, did not cause lysis or antibody staining of aa, cc, and gg PBMCs, and precipitated antigens of M r 45 000 and 12 000 (Fig. 1B). In addition, both mAbs reacted only with PD14-transfected L cells. Hence, these two mAbs react with polymorphic determinant (s) of class I antigen (s) and recognize the PD 14 gene product. Unlike mAb 74-11-10, these new mAbs recognize a potentially unique SLA d class I determinant (s), both as the dilute culture supernatants and as the more concentrated ascites. At this moment we are unable to determine whether these two mAbs react with products of other class I genes. This determination will have to await the successful expression of each and every SLA class I gene in appropriate tissue culture cells. If either or both of these mAbs recognize a unique determinant of the PD14 gene product, then they will represent the first mAbs reactive with a private SLA specificity. For this reason, they are now being tested in SLA typing laboratories in France, Switzerland, Sweden, and the US to see whether known private SLA specificities are being detected. mAb 2.28. 1 (IgM) is reactive with cells bearing all three SLA haplotypes and reacts only with PDl-trans-fected L cells, indicating that it may recognize a monomorphic determinant of a single locus product (Fig. 1B). If it can be shown that this mAb is unreactive with the products of other SLA class I genes, it will potentially be a very useful mAb for determining the locus reactivity of other antibodies. Lysostrip analyses, binding inhibition studies, and/or immunochemical studies using this mAb in conjunction with known swine SLA typing reagents (Vaiman et al. 1979; Renard et al. 1988) could indicate whether the typing serum reacts with the PDI locus product. However, this mAb will only be useful here if it can be proven to react with the same specific locus product in every haplotype. For human class II mAbs it has clearly been shown that some mAbs specific for one locus in one haplotype can cross-react with other locus