A study was carried out to determine the effect on lymphocytic cortisol metabolism (LCM) of plasma from 62 patients with diffuse thyrotoxic goitre (DTG), 14 patients with toxic nodular goitre (TNG) and ten hypothyroid patients. Plasma of 33 healthy donors served as controls. A known concentration of human lymphocytes was incubated with cortisol in media containing 50% phosphate-buffered saline (PBS) and 50% of one of the following additions: (1) PBS, (2) homologous plasma (HP), (3) heterologous plasma, (4) plasma from DTG patients, (5) plasma from TNG patients, (6) plasma from hypothyroid patients, (7) PBS and HP to which L-thyroxine (T4) and triiodothyronine (T3) had been added separately and as a mixture up to a concentration ten times the normal, (8) boiled HP and (9) boiled DTG plasma. Plasma from hypothyroid patients gave an LCM-enhancing effect (LCMEE) similar to that of HP. The plasma of DTG and TNG patients had a markedly lesser effect on LCM than did HP. The T4 and T3 had no additional effect when added to PBS or HP. Boiling of HP and DTG plasma resulted in a similar decrease in LCMEE. The findings of this study raise the possibility of the existence of a factor inhibiting LCMEE in the plasma of thyrotoxic patients.