The in vitro effects of long-acting thyroid stimulator (LATS) and thyrotrophin (TSH) on phosphodiesterase activity in human thyroid homogenates were studied. The enzyme activity was estimated by the destruction of 3H-cyclic 3′,5′-adenosine monophosphate. Bovine TSH had no effect on the enzyme activity. Five experiments were carried out using different batches of LATS-IgG and normal IgG which were preincubated with thyroid homogenates or not. There was no significant difference between the enzyme activity with LATS-IgG and that with normal IgG, while the activity was mostly inhibited by theophylline. These data are not consistent with the hypothesis that LATS acts as an antibody to phosphodiesterase.