Among the transition and group II elements the abundance of zinc in biology is second only to that of iron. It is present in and indispensable to all forms of life. Knowledge regarding the biological occurrence and function of zinc, the development of methods that permit its detection and those aspects of biochemistry which have allowed an appreciation of the manner in which it participates in generating biological specificity have grown exponentially in the last two decades. The recognition that zinc is an essential, coenzyme-like component of many enzymes, indispensable to their function, is reminiscent of the role of some vitamins. Technical advances in the isolation and characterization of enzymes in general and metalloenzymes in particular were basic to the rapid growth of zinc metallobiochemistry during the last two decades. Zinc is now known to be an integral component of a large variety of proteins and enzymes and, hence, the total zinc content of tissues cannot serve as a guide to the multiple functions of the metal. Zinc participates in a wide variety of metabolic processes including carbohydrate, lipid, protein and nucleic acid synthesis or degradation. Most recently, zinc proteins have been shown to be involved in the transcription and translation of the genetic material, perhaps accounting for its essentiality to all forms of life. A number of zinc proteins have been described whose function is not known as yet. Among these, the metallothioneins are prominent. They are widely distributed, of low molecular weight (approximately 6700), and their content of cadmium and/or zinc as well as sulfur is exceptionally high.(ABSTRACT TRUNCATED AT 250 WORDS)