From 1984 through 1986, a total of 20 patients with biopsy confirmed nasopharyngeal carcinoma (NPC) were randomly selected for studies on natural killer cell (NK) activity before and at the end of radiotherapy. Lymphocyte separation was done by centrifuge of heparinized peripheral blood with Ficoll-Hypaque and monocytes were removed by Petri dish adhesion. K562 cells were used as target cells of NK activity. The lymphocyte-target reaction of 4 hours was done and the ratio was 50:1, 25:1, and 12.5:1. The results of 50:1 was taken as the NK activity. The mean value of NK activity in NPC patients after radiotherapy was 43.0 +/- 21.1%, this is not significantly different from 41.2 +/- 23.6% before radiotherapy. On the other hand, before radiotherapy, the patients with too high NK activity (greater than 40%) or too low NK activity (less than 20%) had a mild tendency toward median value (20%-40%) after radiotherapy.