The objective of the present study was to test the reliability of the analysis method of the reflectance spectrum of fat in the zone of light absorption by carotenoid pigments developed in lambs to trace grass feeding in calves. Six feeding treatments were compared in suckling calves: ad libitum cocksfoot hay plus ad libitum concentrate or half level of ad libitum concentrate (HH and HL), ad libitum cut grass plus ad libitum concentrate or half level of ad libitum concentrate (GH and GL), ad libitum highland grazed grass plus ad libitum concentrate (PH); ad libitum medium quality cocksfoot hay plus dam's milk (C0). Groups HH, HL, GH, GL and PH were slaughtered at approximately 10 months of age, after 150 days of experimental feeding diet. C0 calves were slaughtered at the beginning of the experiment at approximately 6 months of age. The concentration of carotenoids was analysed by HPLC on diets and calves' plasma at slaughter. The reflectance spectrum of perirenal and subcutaneous fat were measured at slaughter and expressed as AVI units. Plasma carotenoid content was higher (P<0·05) in PH and GL calves than in HH, HL and GH calves. There was an effect (P<0·05) of feeding treatment and measurement site on AVI. Values obtained for perirenal fat were higher (P<0·05) than those obtained for subcutaneous fat. For plasma carotenoid content and for AVI values in both fat tissues there was no overlapping between hay-fed calves on the one hand and GL or grazing calves on the other hand. However, there was some overlapping between hay-fed calves and GH calves. For plasma carotenoid content, values for C0 calves overlapped with values of GH and GL calves. For both fat tissues AVI values of C0 calves overlapped with values of HH, HL, GH and GL calves. AVI of cut grass fed calves (GH and GL groups) increased linearly (P<0·05) with average daily grass intake for both tissues considered. AVI in subcutaneous fat tissue decreased linearly (P<0·05) with average daily concentrate intake but this relation was not significant for AVI in perirenal fat (P>0·05). Plasma carotenoid concentration and the reflectance spectrum of the fat in the zone of light absorption by carotenoid pigments could be used to trace grass feeding in calves. Results obtained in this study suggest that different fatty tissues could differ in their reliability and interest for traceability of grass-feeding, depending on the animal age and production system.