The PITSLRE protein kinases, hereafter referred to as CDK11 because of their association with the cyclin L regulatory partner, belong to large molecular weight protein complexes that contain RNA polymerase II. These CDK11p110 complexes have been reported to influence transcription as well as interact with the general pre-mRNA-splicing factor RNPS1. Some of these complexes may also play a role in pre-mRNA splicing. Using a two-hybrid interactive screen, the splicing protein 9G8 was identified as an in vivo partner for CDK11p110. The identification of several splicing-related factors as CDK11p110 interactors along with the close relationship between transcription and splicing indicated that CDK11p110 might influence splicing activity directly. Immunodepletion of CDK11p110 from splicing extracts greatly reduced the appearance of spliced products using an in vitro assay system. Moreover, the re-addition of these CDK11p110 immune complexes to the CDK11p110-immunodepleted splicing reactions completely restored splicing activity. Similarly, the addition of purified CDK11p110 amino-terminal domain protein was sufficient to inhibit the splicing reaction. Finally, 9G8 is a phosphoprotein in vivo and is a substrate for CDK11p110 phosphorylation in vitro. These data are among the first demonstrations showing that a CDK activity is functionally coupled to the regulation of pre-mRNA-splicing events and further support the hypothesis that CDK11p110 is in a signaling pathway that may help to coordinate transcription and RNA-processing events.