Soluble class I molecules were immunoprecipitated from human sweat and serum using the BB7.7 monoclonal antibody (mAb) coupled to immunomagnetic beads. Molecules were analysed biochemically on SDS-PAGE gels and finally by ID-isoelectric-focusing (IEF). Serum- and sweat-HLA IEF-band patterns of the same individual were fully identical, showing that HLA excreted in sweat possess polymorphic structures like those in serum. Quantitatively, we used a highly sensitive competitive enzyme-linked immunosorbent (ELISA) assay to determine soluble class I concentrations. The first group was that of non-HLA-A9 and -Bw62 sera, which were found to contain HLA levels with a mean concentration of 0.82 +/- 0.63 microgram/ml (n = 44). However, sera that were HLA-A23 or -24 (splits of HLA-A9) contained higher levels, with a mean of 3.2 +/- 0.94 microgram/ml (n = 20). Similarly, HLA-Bw62 individuals had a higher mean of 2.05 +/- 0.65 micrograms/ml (n = 10). The difference of the HLA-A9 group to the first group was statistically highly significant, P less than 0.0001, and that of the HLA-Bw62 to the first was also significant, P less than 0.004. Individuals who were both HLA-A9 and -Bw62 (n = 5) did not express significantly higher levels than those who only had one of these specificities. Sweat HLA levels had a mean of 0.42 +/- 0.4 microgram/ml (n = 10). These results show for the first time that soluble class I peptides are excreted in relatively high concentrations in sweat and possess polymorphic structures identical to those of serum HLA and that serum HLA levels are allotype dependent.