RECENT data on galactic redshift anomalies have led Hoyle and Narlikar1, for example, to propose a qualitative explanation in terms of spatio-temporal variation of proper mass. The consequences of hypotheses such as this are evidently worthy of study, but there are other redshift anomalies, which should also be considered in this context. A possible alternative explanation is in terms of an inelastic photon–photon interaction; this has the advantages of being more inclusive, more quantitative (at least in order of magnitude) and immediately testable in the laboratory.