Zinquin (ZQ) is a commonly used sensor for cellular Zn2+ status. It has been assumed that it measures accessible Zn2+ concentrations in the nanomolar range. Instead, this report shows a consistent pattern across seven mammalian cell and tissue types that ZQ reacts with micromolar concentrations of Zn2+ bound as Zn-proteins. The predominant class of products were ZQ:Zn-protein adducts that were characterized in vivo and in vitro by a fluorescence emission spectrum centered at about 470 nm, by their migration over Sephadex G-75 as protein not low molecular weight species, by the exclusion of reaction with lipid vesicles, and by their large aggregate concentration. In addition, variable, minor formation of Zn(ZQ)(2) with a fluorescence band at about 490 nm was observed in vivo in each case. Because incubation of isolated Zn-proteome with ZQ also generated similar amounts of Zn(ZQ)(2), it was concluded that this species had formed through direct ligand substitution in which ZQ had successfully competed for protein-bound Zn2+. Parallel studies with the model Zn-proteins, alcohol dehydrogenase (ADH), and alkaline phosphatase (AP) revealed a similar picture of reactivity: ZQ(ACID) (Zinquin acid, (2-methyl-8-p-toluenesulfonamido-6-quinolyloxy)acetate)) able to bind to one Zn2+ and extract the other in Zn-2-ADH, whereas it removed one Zn2+ from Zn-2-AP and did not bind to the other. Zinquin ethyl ester (ethyl(2-methyl-8-p-toluenesulfonamido-6-quinolyloxy)acetate); ZQ(EE)) bound to both proteins without sequestering Zn2+ from either one. In contrast to a closely related sensor, 6-methoxy-8-p-toluenesulfonamido-quinoline (TSQ), neither ZQ(ACID) nor ZQ(EE) associated with Zn-carbonic anhydrase. A survey of reactivity of these sensors with partially fractionated Zn-proteome confirmed that ZQ and TSQ bind to distinct, overlapping subsets of the Zn-proteome.