Since 1978 the National Toxicology Program (NTP) has coordinated, developed, carried out, and informed the public about toxicology testing within the federal government. For chemical carcinogenesis bioassays, the NTP has long used a set of standardized criteria for classifying the relative strength of experimental outcomes. These criteria have enabled the NTP to assess the potential carcinogenicity of nearly 600 substances. Now the program is launching similar criteria for assessing other types of adverse effects, such as damage to the immune system, the reproductive system, and the developing organism. The new criteria attempt to impose a sense of uniformity to different types of toxicology data. “It’s very difficult to develop criteria for noncancer end points,” says Nancy Kerkvliet, a professor of immunotoxicology at Oregon State University, a former member of the NTP Board of Scientific Counselors (BSC), and chair of the Working Group that reviewed the new immunotoxicity classification criteria. “There’s such a variety of end points. It’s difficult to rank them for importance.” The NTP, aware of this inherent difficulty, hopes the levels-of-evidence criteria will allow better transference to public health decisions. Decades of cancer studies have provided a widely accepted template for assessing and evaluating the relative comparability and importance of various cancer study outcomes. First created in 1983, the NTP’s 5-tiered classification system describes the strength of evidence for conclusions for NTP chemical carcinogenesis studies. These classifications—which include 1) clear evidence, 2) some evidence, 3) equivocal evidence, 4) no evidence, and 5) inadequate study—are used to frame the conclusions per sex/species in the experiment cited in each NTP technical report. “These classifications have stood the test of time,” says Paul Foster, NTP discipline leader for reproductive and developmental toxicology. The strongest findings, which can be considered “clear evidence” of carcinogenic activity, are demonstrated by studies that are interpreted as showing a dose-related increase of malignant neoplasms, a combination of malignant and benign neoplasms, or benign neoplasms if there is any indication that such tumors could progress to malignancy. To help NTP staff decide on borderline cases, there are 15 additional reference points, or factors to be considered in addressing various aspects, such as “presence or absence of dose relationships” and “statistical significance of the observed tumor increase.” Using these criteria allows NTP staff to better develop consistent conclusions for multiple studies of the same substance and for comparing different chemicals under these same criteria. The NTP classification system is applied only to those studies conducted by the program. The 5 designations are intended to be solely a conclusion about whether a substance may potentially pose a carcinogenic hazard to humans. They are not intended to describe a carcinogenic risk to human health, but to be an integral part of the more formal risk assessment process that takes place at state and other federal agencies. This formal risk assessment requires additional evaluation of factors such as specific doses, routes, and likelihood of exposure as well as a whole host of other information including published bioassays and toxicology studies conducted by different organizations.