Phosphofructokinase (PFK; ATP: D-fructose-6-phosphate transferase, EC 2.7.1.11) was studied in human thyroid carcinomas (n = 16), follicular adenomas (n = 31) and normal thyroid tissue (n = 19). The specific activity in carcinomas (0.129 +/- 0.070) is significantly increased (p less than 0.001) in comparison with phosphofructokinase in normal thyroid tissue (0.028 +/- 0.009). No difference in phosphofructokinase activity seems to exist between follicular, papillary and undifferentiated carcinomas. Specific activities of follicular adenomas are rather heterogeneous. When these tumors were divided into three groups of increasing proliferative activity as judged by histopathological criteria, highest specific activities of phosphofructokinase were found in the group with the highest proliferative activity. The latter group resembles the enzyme activities found in carcinomas. On the other hand specific enzyme activities of the least active tissues were comparable to normal and different from carcinomas. Adenomas show the same isozyme composition as found in normal thyroid tissue. All three isozymes of PFK, M-(muscle)type, L-(liver)type and P-(platelet)type, were present. Phosphofructokinase from papillary carcinomas show a lesser degree of precipitation with anti-M antibodies. A higher amount of platelet type isoenzyme is found in papillary carcinomas compared to follicular and undifferentiated carcinomas. The influence of citrate, an inhibitor of phosphofructokinase, is in agreement with the isozyme composition.