133Xe clearance method has been employed for measurement of the blood flow in 6C3HED lymphosarcoma implanted in flank of C3H mice. In small tumors (9-day old) the mean rate of blood flow was 9.4 +/- 5.0 ml/100g/min. but with progressing growth of tumors the rate of blood flow significantly decreased. In large tumors (14-day old) the mean rate of the blood flow was only 5.4 +/- 2.8. Our results demonstrate that this type of tumor, similarly as most of transplanted tumors are not well perfused and, that there is an inverse relationhip between the blood flow and the tumor size.