An in vitro study has been conducted in order to improve the understanding of the interaction between lymphocytes and inplant materials. The uptake of cellular thymidine for DMA synthesis was used to monitor the cellular responses, in conjunction with microscopy. Material ellutions stimulated the uptake of thymidine by cells, whereas solid material caused an inhibition of thymidine uptake. This inhibition was seen both in PHA-stimulated cultures and in cultures in the presence of materials only. Inhibition of thymidine uptake was found to be caused indirectly by contact with material surfaces and directly by an active cell secretion, produced in response to the materials. It was postulated that this active cell secretion was a lymphotoxin.