Cyclin-dependent kinase-5 (cdk-5) is a serine/threonine kinase that displays neurone-specific activity. Experimental manipulation of cdk-5 expression in neurones has shown that cdk-5 is essential for proper development of the nervous system and, in particular, for outgrowth of neurites. Such observations suggest that cdk-5 activity must be tightly controlled during development of the nervous system. To identify possible regulators of cdk-5, we used the yeast two-hybrid system to search for proteins that interact with cdk-5. In two independent yeast transformation events, cyclin D2 interacted with cdk-5. Immunoprecipitation experiments confirmed that cyclin D2 and cdk-5 interact in mammalian cells. Cyclin D2 did not activate cdk-5 as assayed using three different substrates, which was in contrast to a known cdk-5 activator, p35. However, cyclin D2 expression led to a decrease in cdk-5/p35 activity in transfected cells. As cyclin D2 and cdk-5 are known to share overlapping patterns of expression during development of the CNS, the results presented here suggest a role for cyclin D2 in modulating cdk-5 activity in postmitotic developing neurones.