A method was developed for the simultaneous determination of Ce, Nd and Sm in biological materials at sub- or low ng g^–1 levels by isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). The monitored isotopic pairs for sample analysis were ¹⁴⁰Ce/¹⁴²Ce, ¹⁴³Nd/¹⁴⁶Nd and ¹⁴⁷Sm/¹⁴⁹Sm. The isobaric interference of ¹⁴²Nd on ¹⁴²Ce was corrected by measuring ¹⁴³Nd and calculating the percentage contribution of ¹⁴²Nd to the ¹⁴²Ce analyte peak. The total mass bias was determined experimentally and corrected by use of a standard solution of natural abundances. The overall performance of the procedure was checked by analysing standard solutions of natural Ce, Nd and Sm. The recoveries of Ce, Nd and Sm at the 10 ng level were 99.7, 103 and 102% with precisions of 1.4, 1.5 and 2.6% RSD, respectively. The validity of the analytical procedure was further examined by analysing a certified reference material (CRM) (Bush Leaves, GBW 07603, China). The results were in good agreement with the certified values of the CRMs (all results fell within the specified uncertainties), with RSDs of 6.3–8.5%. The method detection limits for Ce, Nd and Sm were 0.55, 0.17 and 0.10 ng g^–1, respectively. The method was used to determine Ce, Nd and Sm in two Chinese CRMs of Human Hair (GBW 09101) and Wheat Flour (GBW 08503). The results obtained by ID-ICP-MS agreed well with those obtained by external calibration ICP-MS from four laboratories; the deviations of Ce, Nd and Sm for Human Hair were 0.5, 3.4 and 1.9%. and for Wheat Flour 1.4, 4.7 and 10.1%, respectively. A t-test indicated that the results determined by ID-ICP-MS showed no significant difference from those obtained by external calibration ICP-MS (P > 0.05), except for Sm in Wheat Flour (P < 0.05). The results were also compared with those of an NAA method. The deviations of Ce, Nd and Sm for Human Hair were 2.1, 19.7 and 4.1%, and for Wheat Flour 3.7, 14.0 and 18.0%, respectively. A t-test indicated that the results for Ce and Sm in Human Hair and Ce in Wheat Flour showed no significant difference between ID-ICP-MS and NAA (P > 0.05); however, for Nd in both samples and Sm in Wheat Flour there were significant differences between ID-ICP-MS and NAA (P < 0.05). A z-score assessment program was carried out following similar procedures to those used in the International Proficiency Test for Analytical Geochemistry Laboratories. The results indicated that all z-score results for Ce, Nd and Sm for both Human Hair and Wheat Flour samples were in the range –2 < z < 2. Hence, the analytical results of the present work were considered to be satisfactory.