In vitro toxicity testing has found widespread application in its use for screening materials for medical devices. Cytotoxicity tests, which have been in use for nearly 20 years, have been validated for intralaboratory repeatability, interlaboratory reproducibility, and correlation with acute animal toxicity assays. The three primary cytotoxicity assays, i.e., direct contact, agar diffusion, and elution tests, allow a selection between assay and material characteristics. Mutagenicity assays have had limited application to materials testing because of the insoluble nature of the materials and the low level of extractable chemicals, which are generally below the sensitivity limit of these assays. In vitro blood compatibility tests for hemolysis and complement activation are used primarily for blood contacting materials in applications where there is a large surface area of material for ex vivo applications.