Transformed human kidney cells (293 cells) exposed to 12.5 to 37.5 microM CdCl(2) showed apoptosis as confirmed by characteristic electron microscopic features, a ladder on gel electrophoresis of extracted DNA, and fragmentation of nucleosomes as detected by enzyme linked immunosorbent assay (ELISA). Higher concentrations of Cd were less effective in inducing apoptosis. Furthermore, addition of the protein extract from the serum-free medium used for Cd-exposure promoted apoptosis exhibiting the same features as that after Cd-exposure. The apoptosis induced by the protein was dose-dependent. The molecular weight of the protein (Cd-protein) was shown to be 40 kDa by gel filtration. Two-dimensional electrophoresis revealed the Cd-protein as a single spot with a molecular weight of 6 kDa and pI of 4.5. Competitive ELISA showed that the Cd-protein reacted with anti-metallothionein antibody. The present findings suggest that apoptosis is induced not only by Cd itself but also by polymerized metallothionein molecules (MT) released from cells into the medium.