Copper uptake and distribution with time among cytoplasmic proteins were followed in cultured cells under several conditions: (1) CHO cells, which cannot synthesize metallothioneins, were labeled with(67)Cu in the presence of 100 μM ZnCl2; (2) Cd(r)30F9 cells, which contain some constitutive metallothionein (MT), were labeled in the absence of additional ZnCl2 and; (3) Cd(r)30F9 cells were labeled in the presence of ZnCl2, under which conditions they synthesized additional metallothioneins. The exogenous(67)Cu and ZnCl2, where present, were then removed, and the distributions of(67)Cu among size fractions of the cellular proteins were observed at intervals for 16 h. In addition, a culture identical to condition (3) above was also treated with 100 μM ZnCl2 during the redistribution period. The(67)Cu was initially resolved into three peaks by Sephadex G-75 chromatography: high molecular weight, intermediate molecular weight, and MT. The(67)Cu in the MT fraction decreased with at 1/2 of 10-12 h. In contrast to this, generally, in cells with a higher initial(67)Cu bound to metallothionein, there was a progressive increase in the amount of(67)Cu eluting with the high- and intermediate-molecular-weight fractions. Since no other source of(67)Cu was available, these experiments suggest that copper stored in MT can be transferred to other proteins in these cells.