The "in vitro" immune effects of different Ti compounds on peripheral blood mononuclear cells (PBMC) were determined. The results show that Ti dioxide is not immunotoxic; Ti oxalate is strongly immunotoxic; titanocene inhibits cytokine release but not PBMC proliferation, while Ti ascorbate inhibits TNF-alpha release from PBMC but not that of IFN-gamma. The results show that Ti toxicity depends on speciation.