Instrumental neutron activation analysis was applied to determine La, Ce, Nd, Sm, Eu, Tb, Yb, Lu and Sc in two biological
reference materials: NIST 1575 Pine Needles and BCR-CRM 101 Spruce Needles. The purpose was to contribute to the reference
data for these two reference materials. The results were obtained with a good precision (relative standard deviations less
than 15%). For the Pine Needles reference material there are already some proposed values and our results showed, in general,
a good agreement with the data published. The contribution of uranium fission products to La, Ce, Nd and Sm was evaluated
and considered in the determination of these elements. Interferences in the determination of rare earth elements in biological
materials are also discussed.