In this paper we develop an approach to interactive verification of programmable logic controllers which often serve as controllers in safety critical systems and hence need thorough verification. As a verification tool we use the KIV system, whereas the formalization is done in modal Kleene algebra. We first prove a bunch of theorems from modal Kleene algebra in KIV, subsequently translate the desired properties of a program for a programmable logic controller in modal Kleene algebra, and finally prove these encoded properties interactively with KIV.