Serum amyloid A (SAA) and acid phosphatase (AcP) levels were determined in serial serum samples of 35 patients in different stages of dissemination and correlated with activity of carcinoma of the prostate. Up to 500-fold increases in SAA level were detected during active periods of cancer with a decrease towards the normal range in remission, in comparison with a 10-fold increase of AcP. The correlation between these two parameters was highly significant (P<0.001), but="" while="" saa="" shows="" 100%="" sensivity="" during="" the="" active="" stage,="" acp="" shows="" only="" 85%="" sensitivity.="" it="" is="" suggested="" that="" althouth="" saa="" is="" not="" a="" specific="" marker="" for="" any="" particular="" illness,="" due="" to="" its="" characteristic="" pattern="" of="" change="" in="" malignant="" diseases="" and="" its="" high="" sensitivity,="" it="" represents="" a="" useful="" biochemical="" parameter="" for="" the="" assessment="" of="" the="" activity="" of="" the="" disease="" to="" monitor="" response="" to="" therapy="" during="">