After Ctr1-mediated copper ion (Cu) entry into the human cytoplasm， chaperones Atox1 and CCS deliver Cu to P1B-type ATPases and to superoxide dismutase， respectively， via direct protein-protein interactions. Although the two Cu chaperones are presumed to work along independent pathways， we here assessed cross-reactivity between Atox1 and the first domain of CCS (CCS1) using biochemical and biophysical methods in vitro. By NMR we show that CCS1 is monomeric although it elutes differently from Atox1 in size exclusion chromatography (SEC). This property allows separation of Atox1 and CCS1 by SEC and， combined with the 254/280 nm ratio as an indicator of Cu loading， we demonstrate that Cu can be transferred from one protein to the other. Cu exchange also occurs with full-length CCS and， as expected， the interaction involves the metal binding sites since mutation of Cu-binding cysteine in Atox1 eliminates Cu transfer from CCS1. Cross-reactivity between CCS and Atox1 may aid in regulation of Cu distribution in the cytoplasm.