Proliferative responses of peripheral blood mononuclear cells (PBMCs) to stimulation with parent and hybrid cells. (a) Proliferative responses of PBMCs from Responder 1, a normal allogeneic, Epstein–Barr virus (EBV)‐seronegative donor to stimulation with 3·3×104 mitomycin C‐treated parent EBV transformed B‐lymphoblastoid cell line (B‐LCL) (HMy2) cells, parent tumour cells (HL60, CEM, K562), or EBV B‐LCL/tumour cell hybrid clones (HMy2×HL60, HMy2×CEM, HMy2×K562) in mixed lymphocyte/tumour cell microcultures (MLTC) in vitro. Cell proliferation is expressed in terms of [3H]thymidine incorporation (mean ± SD, n = 3). (b) Proliferative responses of PBMCs from Responder 4, a normal, allogeneic, EBV‐seropositive donor, to stimulation, as described above.