BHK-21 (C-13) cells were exposed to freshly mixed zinc phosphate cement for 0.5, 1 and 2 h in a simulated cavity apparatus. The effect on the cells was evaluated by quantitative measurement of acid phosphatase and succinic dehydrogenase reaction product in order to determine organelle membrane permeability. After 0.5 h cell-material contact there was no significant effect. After 1 h cell-material contact, there was slight labilization of the lysosomal membranes, and a significant decrease in succinic dehydrogenase activity. The mean overall stain density in the experimental cells was 109% and 57% of the control values for acid phosphotase and succinic dehydrogenase respectively. After 2 h cell-material contact, there was significant labilization of the lysosomal membranes, and also labilization of the mitochondrial membranes. The mean overall stain densities were 114% and 75% of the control values for acid phosphatase and succinic dehydrogenase respectively. The relevance of these findings to the in vivo situation is discussed.