A panel of 53 antibodies from the ISOBM TD-3 PSA Workshop were tested for reactivity with iodinated derivatives of free PSA or the alpha1-antichymotrypsin PSA complex using the BIAcore system. Two antibodies (#69 and #83) showed low binding (<8%) for both antigens. One group of antibodies (#25, #26, #33, #54, #68, #73, #77, #78 and #91) had a much lower affinity for the complex (<12%) than for the free antigen (>65%). According to the mapping study, it was possible to categorize the antibodies into 29 different groups. Four antibodies were not classified. The two-dimensional representation of all interactions between the antibodies showed a complex network on the PSA molecule. Antibodies with lower affinity for the complex than for the free PSA appeared to bind epitopes in a common region, and thus it was not possible to perform sandwich assays with antibodies specific for free PSA.