Methods are currently being explored which would permit the application of cytogenetic procedures such as metaphase and anaphase analyses to in vivo mammalian systems in order that due consideration may be given to metabolic activation or detoxification reactions which might affect the mutagenic or clastogenic (chromosome breaking) potential of environmental agents under study. It has been recommended (1) that such in vivo cytogenetic methods be employed in mutagenicity testingf programs which are being designed to evaluate mutagenic potentials and riskbenefit ratios of a wide variety of drugs and other chemical agents when human exposure is involved. Metaphase chromosome analysis has been adapted readily to in vivo testing of chemical mutagens (2-5). More recently, Palmer, Kelly-Garvert, and Legator (6) have described a potentially useful method for in vivo collection of anaphase figures from rat bone marrow cells which involves the application of a mitotic arresting agent (colcemide, CIBA, New Jersey) in vivo, followed by a short-term in vitro incubation (lll/' hr) in the absence of colcemide to reconstitute spindle fibers and initiate anaphase movement. Preliminary studies with the in vivo anaphase method in our laboratory have revealed a number of technical limita-