A man with a prostate specific antigen (PSA) of 6.1 ng/mL, a clinical stage T2b prostate nodule and biopsies that showed Gleason sum 6 adenocarcinoma of the prostate underwent a radical prostatectomy. The final pathology showed organ-confined disease. His postoperative PSA remained elevated at 4.0 ng/mL. The PSA was repeated several times and was in the same range. It was re-evaluated at another lab facility and was unmeasurable (<0.02 ng/mL). He has an antibody that cross-reacts with an assay reagent causing this false reading. The most likely antibody is one against mouse immunoglobulin G (IgG).