The phenotypes and gene frequencies of three serum protein systems--Hp, GC and C3--were studied in 184 consecutive patients from all over Greece with colon cancer. Healthy Greeks studied previously in our department served as controls. No significant differences were found between patients and controls concerning GC and C3. Significant differences were found in the Hp system; the frequencies of the Hp*1 gene and the Hp 1-1 phenotype were significantly higher in patients than in controls.