The 64Cu production in biological tissues after reactor neutron activation was compared (a) in different types of tissue and (b) in different sizes of samples. The comparison showed that composition of animal tissue should be taken into account since it is a source of interference during neutron activation of copper. Undirectional deviations of 10% depending on sample type and size can be caused by this interference. The copper content in pooled tissues of Lymnaea stagnalis is an example. If properly corrected, these were 21 µg dry intestinal tissue and 5.7 µg / g shell.