Biochemical kinds such as proteins pose interesting problems for philosophers of science. They can be studied both from the point of view of biology and chemistry, but these different perspectives may result in different classificatory practices. I will examine the tension that such classificatory differences produce. The reducibility of the biological functions of biochemical kinds to the chemical structures that realize these functions is a key question here. This leads us to a more general discussion of multiple realizability and realization at the biology-chemistry interface. On the face of it, the case of biochemical kinds motivates a dual theory of chemical and biological kinds, hence a type of pluralism. But it will be argued that a dual theory is not necessary and genuine multiple realization at this interface may be rare. I conclude with a defence of natural kind monism.