Study of interdisciplinary research processes and performance is hampered by a lack of data. This project investigated possible indicators based in the open scientific literature to measure such processes. Focusing on theJournal Citation Reports as a suitable data base, alternative indicators were validated on a sample of 383 articles drawn from 19 journals. The results support the use ofCitations Outside Category as an indicator of cross-disciplinary research activity. An estimated version of this indicator is used to examine three research categories — Demography, Operations Research/Management Science, and Toxicology — as to the extent of cross-disciplinary citation occurring by the journals in these categories and to them. Results suggest thatCitations Outside Category can be a quite informative bibliometric measure. A key substantive finding is that citation across broad field categories (engineering, life sciences, physical sciences, and social sciences) is extremely infrequent.