The effect of in vitro irradiation with increasing in logarythmic progress X-ray doses on lymphocyte viability and on T and B lymphocyte populations was studied in normal adults, patients with myasthenia gravis and in patients undergoing long-term steroid therapy. Decrease in numbers of lymphocytes carrying T or B lymphocyte surface markers was higher than viable cell loss. The decrease showed no linear correlation with X-ray doses applied, which might reflect the existence of radioresistant T and B lymphocytes. A higher so called early radiosensitivity of B lymphocytes was demonstrated. In patients with myasthenia gravis early radioresistance of T lymphocytes was detected. In patients undergoing long-term steroid therapy, an increase in numbers of cells lacking markers of any of lymphocyte populations was found in parallel with a decrease in T lymphocyte number which, in these patients, showed a higher radiosensitivity.