SUMMARYUsing an anti‐Pkmonoclonal antibody (mAb) designated CPK‐1, the expression of the Pkantigen was assessed on normal human tissue from non‐Pkindividuals. Although the Pkantigen was detected on fibroblasts and blood vessels as previously reported, it was also found on smooth muscle cells of the digestive tract and the urogenital system. Pkwas also found on glandular cells of the stomach, oesophagus and prostate. Additionally, CPK‐1 reacted weakly with oesophagus squamous cells, and a small number of glomeruli and tubules in the kidney. The mechanism of expression of the Pkdeterminant in non‐Pkindividuals is