Studies on particle retention time in ruminants are commonly conducted utilizing chromium (Cr) and/or a lanthanide bound to feeds. Both types of markers have different chemical properties which potentially bias estimates of digesta retention—their combined use is therefore a topic of ongoing discussion. In order to evaluate the suitability of two lanthanides for studies assessing the passage kinetics of different-sized particles, we measured the mean retention time in roe deer with cerium (Ce) and lanthanum (La) labelled to the same particle length and compared it to Cr-mordanted fibre of the same size. We expected a simultaneous excretion of Ce- and La-labelled fibre, but a delayed excretion of Cr-mordanted fibre compared to the other markers. In this study, the mean retention times of Ce- and La-labelled fibre did not differ significantly, and Cr-mordanted fibre was retained on average 5 h longer than Ce and La. Despite the limitation of the small sample size, Ce and La showed the same excretion pattern and can therefore be recommended as mordants in studies assessing passage kinetics of different particle sizes.