K-theory for triangulated categories 3 1/2 (A): A detailed proof of the theorem of homological functors Amnon Neeman Abstract. Let $\mathcal A$ and $\mathcal B$ be abelian categories. Let $H:\mathcal A\rightarrow\mathcal B$ be a bounded $\delta$-functor. We prove that $H$ induces a natural map in higher K-theory. From a more precise analysis of the proof, we deduce that it is possible to define a K-theory of the bounded derived category of $\mathcal A$, which contains Quillen's K-theory of $\mathcal A$ as a retract.