The main goal of this study was to use state-of-the-art instruments for nanoparticle (nanoparticle tracking analysis and resonant mass measurement) and microparticle counting (flow imaging) to assess the effects of peristaltic filling pump operation on particle formation in formulations of intravenous immunoglobulin. Microparticle levels were also measured with light obscuration. Postpumping agitation was studied as an accelerated degradation method, 3 different commercial peristaltic tubing types were tested, and the effects of formulation pH and inclusion of polysorbate 80 were determined. Overall, the results documented that nanoparticle measurements, as well as microparticle determinations with flow imaging, were essential to gain rigorous insights into impacts of processing and formulation parameters on pumping- and agitation-induced particle formation. In addition, light obscuration was a relatively insensitive method and failed to detect large increases in protein particles caused by pumping and postpumping agitation. Formulation studies showed that the presence of polysorbate 80 or increasing protein colloidal stability with appropriate choice of buffer generally reduced particle formation. The results highlight the need for filling pump assessments in formulation development studies. Combining such assessments with appropriate analytical methods should help assure that particle levels are controlled during filling pump operation and that the highest quality products are manufactured. Copyright © 2020 American Pharmacists Association®. Published by Elsevier Inc. All rights reserved.