On comparing lymphocyte patterns in patients with polycythaemia vera (PV) versus healthy blood donors (NC) and patients with secondary polycythaemia (SP), we observed an altered CD4/CD8 ratio (p less than 0.01) mainly due to the decreased CD8 subpopulation in PV. We found an increased lymphocyte activity in PV patients when compared with both SP and NC: the interleukin 2 (IL-2) production was significantly higher (p less than 0.001) as was the lymphoproliferative response both to phytohaemagglutinin (p less than 0.001) and IL-2 (p less than 0.005). No significant differences were found between NC and SP. None of the PHA-stimulated PBMC samples from PV, SP or NC produced erythropoietin (EPO)-like or EPO-inhibitory activity. Our findings suggest an altered lymphoid lineage in PV patients.