Both in vivo and in vitro methodologies have uses as cytogenetic test systems for mutagenicity testing. The in vivo methods are certainly the choice as the primary test system, except perhaps in the case of viruses and some biologicals. When an in vitro method is used anaphase offers some advantages as greater speed and ease of observation, the detection of defects produced in G2 and mitosis, and the ability to distinguish between gaps and breaks, while it sacrifices the morphologic detail and ability to localize defects to specific chromosomes found in metaphase.