We evaluated various immunologic parameters in patients with papillary and follicular carcinomas of the thyroid gland. Our studies included examinations of peripheral blood lymphocytes and skin reactions to selected antigens. Preoperative peripheral blood tests were found to be normal except for an elevated percentage of IgG X Fc+ T-cells (T gamma). Skin reactions (phytohemagglutinin, purified protein derivative) were greater preoperatively than postoperatively. In postoperative cases without tumor recurrence, absolute numbers of T-cells and lymphocytes were reduced. OK-432 is a biologic response modifier of a streptococcal preparation and was used as immunotherapy in postoperative patients. This therapy seemed to augment the absolute numbers of T-cells and lymphocytes as well as purified protein derivative skin reactions in the patients without tumor recurrences. In the patients with postoperative tumor recurrences, there was an abnormal reduction in the percentage of T-cells and in the absolute numbers of T-cells and lymphocytes. OK-432 treatment was not significantly effective in normalizing this reduction.