We examined the effect of meta-iodo-benzylguanine (MIBG), an inhibitor of mono (ADP-ribosylation), on growth and clonogenic ability of X-irradiated L5178Y-R (LY-R, radiation resistant) and L5178Y-S (LY-S, radiation sensitive) marine lymphoma cells. We also measured free Ca²⁺ concentration in LY cells and in human lymphocytes, with the fluorescent calcium chelator, Fura-2. We found no radiosensitization by MIBG in either LY subline, whereas free Ca²⁺ concentration increased in MIBG-treated and X-irradiated LY-S cells, as compared to X-irradiated only; in X-irradiated LY-R cells MIBG did not modify the concentration of Ca²⁺. We conclude that there is no direct relation between the level of free Ca²⁺, as determined by fluorescent chelator, and the lethal effect of X-irradiation. We also discuss possible artifacts as reasons of incorrect estimation of intracellular Ca²⁺ concentration.