SUMMARYPeripheral blood leucocytes from healthy subjects (control leucocytes) and from patients with Hashimoto's thyroiditis or idiopathic myxoedema, were incubated with 2‐3‐day‐old human thyroid cells in monolayer cultures. After 5‐6 days the Hashimoto leucocytes appeared to induce more thyroid cell destruction than control leucocytes. Thyroid cell function (measured by cell to medium (C/M) ratios of131I) was reduced in cultures incubated with Hashimoto leucocytes, compared to those incubated with control leucocytes. This effect was not mediated by liberation of thyroid antibodies into the medium, but rather from a direct leucocyte/thyroid cell interaction with a resultant liberation of lysozymes. Preliminary observations indicated that the effect of the Hashimoto leucocytes on thyroid cells could be prevented by prior incubation of the former with anti‐thymocyte globulin; this suggested that sensitized lymphocytes may be directly responsible for the thyroid tissue injury of Hashimoto's disease, perhaps with the adjunctive cooperation of the non‐specific macrophages. Leucocytes from patients with idiopathic myxoedema (with low or absent circulating thyroid antibodies) did not have any effect on the thyroid cells, perhaps because the concentration of peripheral sensitized lymphocytes may possibly have declined in the