Early cytotoxic events were studied on B77 fibroblasts. Cells were treated with sulphur mustard (SM) in short-term experiments in which cell viability was unchanged, as evaluated by the neutral red cytotoxicity test. This treatment was correlated to two early signs of cytotoxicity. The intracellular Ca2+ concentration [Ca2+]i level in SM-treated Fura-2-loaded fibroblasts showed a significant dose-dependent increase. This observed rise was sustained, in contrast to the Ca2+ signal induced by serum, and was already visible 5-10 min after the addition of SM to cell suspensions in vitro. Modification of the extracellular Ca2+ concentration in the medium had no effect on the cytosolic calcium rise caused by SM, suggesting release from intracellular Ca2+ pools. Furthermore, a time-dependent inhibition of the [Ca2+]i transient increase induced by growth-factors (as evaluated by the fetal calf serum (FCS) response) was observed within the first hour of exposure. These latter results suggest that early alterations of calcium distribution induced by SM could be one of the earliest markers of SM intoxication.