We previously reported that LH increased cyclic AMP concentrations in dog thyroid slices and that this effect could not be accounted for on the basis of the stated TSH contamination of the preparation (1). This conclusion now appears incorrect since anti-² subunit LH antibody did not inhibit the effect of 2 μg LH on the thyroid. An equivalent amount of the antibody completely abolished the effect of 2 μg LH on cyclic AMP concentrations in slices of pig corpus luteum. Therefore, the action of LH on the thyroid represented TSH contamination and LH does not have any intrinsic activity to increase cyclic AMP in the thyroid. The TSH contamination of LH appears to be underestimated on the basis of a bioassay dependent upon 32P incorporation into phospholipids in chick thyroid. Bioassay based on cyclic AMP accumulation in thyroid slices indicated a TSH contamination tenfold greater than that reported using 32P incorporation into phospholipids. Two other LH preparations also had greater TSH contamination based on...