This paper pursues the development and empirical exploration of a theoretical framework that explains the probabilities of interactive learning of innovating firms and actors in the public knowledge infrastructure. Our research question reads as follows: To what extent does the strength of innovator firms' internal knowledge resources, the complexity of their innovative activities, and the structuring of their innovative activities influence the probabilities of interactive learning between innovating firms and actors in the knowledge infrastructure? We contend basically that more complex innovative activities increase the probability of internal resource deficits/shortages in innovating firms. The higher the resource deficits/shortages and the lower the alignment of innovative activities, the more likely the search for complementary resources externally, which induces higher probabilities of interactive learning with actors in the knowledge infrastructure. In order to test the generality of our theoretical claims six models were examined, predicting the probability of interactive learning of innovating firms with the knowledge infrastructure (universities and research centres) controlling for sectoral differences in technological dynamics and size effects. Both monotonic and non-monotonic effects were tested. Our findings show that antecedents of patterns of interactive learning differ widely and are contingent upon sectoral technological dynamics and firm size. Our findings enhance a substantial refinement of the main theoretical arguments explaining the level of interactive learning. The absorptive capacity effect turns out to have an inverted U-shape only for small- and medium-sized enterprises (SMEs). The complexity effect is monotonic in some cases and non-monotonic in others. The interaction effect turned out to be U-shaped. The internal integration of innovative activities has no effect at all. Support by bridging institutions had a very strong influence on levels of interactive learning between innovating firms and knowledge infrastructure, but only for SMEs.