Peripheral blood mitogen - induced cellular cytotoxicity (MICC) and natural killer- cell cytotoxicity (NKCC) were assessed in 25 patients with myelodysplastic syndromes (MDS). Both MICC and NKCC were examined under the same experimental conditions using the 18 hr chromium release assay, except that cultures for MICC were stimulated in vitro by the addition of phytohemagglutinin (PHA). Patients' MICC was found significantly reduced, in relation to controls (p less than 0.001), but significantly higher than patients' NKCC (p less than 0.001). Furthermore, patients CD3+ cells and CD4+ cells, as well as the CD4+/CD8+ ratio, were significantly decreased (p less than 0.01, p less than 0.001 and p less than 0.001, respectively), while CD8+ cells and CD16+ cells were within normal limits. No relationship was noted between patients' MICC and total lymphocyte count or any lymphocyte subpopulation. In eleven patients who were subsequently subjected to a-interferon (a-IFN) administration, MICC values were found within normal range one month after the cessation of alpha-IFN, while NKCC values were significantly increased (p less than 0.01), but they still remained below the lower limit of the control (p less than 0.001). Percentages of CD3+, CD4+ and CD8+ cells, as well as the CD4+/CD8+ ratio, did not change after alpha-IFN, but the absolute numbers of CD3+ cells and CD8+ cells were significantly reduced. A statistically significant rise was noted in CD16+ cells. Post- IFN rises in MICC did not correlate with lymphocyte subpopulations. The findings indicate that MDS patients display very low MICC, which can be restored by alpha-IFN administration. The cause of this disturbance and the mechanism of its restoration by alpha-IFN remain unclear.