A key element in the development of a biosimilar molecule is the comparability of the biological activity/nonclinical similarity to the innovator drug. Although some regulatory guidelines are encouraging little or no in vivo testing, currently a common practice is to perform at least one toxicology and/or one pharmacokinetic (PK) study to assess if any different findings occur for in-life, clinical pathology and histopathological parameters or in exposure. An exercise was performed in which the results of such testing were evaluated. It was found that 10 PK comparison studies in the cynomolgus monkey across 4 monoclonal (Mab) classes showed similar exposure in all cases. In 17 toxicology comparison studies with 5 Mab classes performed in the same species and in 7 toxicology comparison studies with non-Mab biosimilars in the rat, no new/unexpected findings were seen and drug exposure measurement gave comparable values in all cases. Overall, although this work does not rule out possible utility of some in vivo testing (notably in the form of stand-alone PK testing) to confirm similar exposure between the 2 molecules tested, it is unclear what benefit can be gained from toxicology testing, especially if comparability has been demonstrated from physiochemical and in vitro characterisation.