The class of organometallic compounds was studied in order to nominate chemicals to the National Cancer Institute as candidates for carcinogen bioassay. A preliminary list of 1,815 candidate chemicals was developed for four major subclasses: (1) alkyl organometallics; (2) aryl organometallics, coordination compounds, and miscellaneous organometallics; (3) organosilicon compounds; and (4) organophosphorus compounds. Based on an evaluation of both the evidence Indicating possible suspicion of carcinogenic activity and evidence indicating potential human exposure, the following five nominations were made: chromium carbonyl, ferrocene, ferric ammonium ferrocyanide, triphenylantimony, and the representative organosilicons, octamethyltrisiloxane, and octamethylcyclotetrasiloxane.