Muscle aldolase bound to muscle FBPase (K-d = 8.7 mu M) decreases the latter's sensitivity towards AMP inhibition. I-0.5 of muscle FBPase was increased from 0.06 mu M to 0.65 mu M when determined in the presence of 10 mu M of muscle aldolase, in the presence of 10 mu M of liver aldolase I-0.5 of liver FBPase was increased only twofold, from 11.0 mu M to 21.7 mu M. The effect of muscle aldolase on liver FBPase and liver aldolase on muscle FBPase is rather negligible. Aldolase slightly affected interaction of FBPase with magnesium ions decreasing K-a and Hill constant (n), No effect of aldolase on FBPase pH optimum was observed.