This paper reports on the preclinical evaluation of merocyanine 540 (MC540) as an agent for the inactivation of tumour cells in BM grafts from non-Hodgkin's lymphoma (NHL) patients. The three cell lines used for this study, OCI-LY13.1, OCI-LY13.2 and OCI-LY9, originate from two patients with high-grade NHL. The OCI-LY13.1 and OCI-LY13.2 lines are derived from the same patient. The OCI-LY13.1 line was established at the time of diagnosis while the OCI-LY13.2 line was established after the tumour had become refractory to therapy. When used under conditions that are known to preserve about 50% of normal human pluripotent hematopoietic progenitor cells (CFU-GEMM), MC540-sensitized photoirradiation reduced in vitro clonogenic OCI-LY9 cells by 4 orders of magnitude and OCI-LY13.1 and OCI-LY13.2 cells by > or = 5. Survival curves for OCI-LY13.1 and OCI-LY13.2 cells were similar and followed first order kinetics, while those for OCI-LY9 cells were distinctly biphasic. Suspension cultures established with photoinactivated lymphoma cells confirmed that MC540-sensitized photoirradiation was cytotoxic and capable of eliminating > or = 4 log of tumour cells. These results encourage the further exploration of MC540-sensitized photoirradiation as a means to purge autologous marrow grafts from NHL patients.