Although the immune response is predominantly controlled at the transcriptional level, microRNA-mediated RNA interference is emerging as an important regulatory mechanism that operates at the translation level. Specifically, recent studies indicate that those miRNAs that are selectively and/or highly expressed in immune cells including the miR-17-92 cluster, miR-150, miR-155, miR-181 and miR-223 have a 'permissive' function in the maturation, proliferation and differentiation of myeloid and lymphoid cells. Importantly, these actions of miRNAs often involve interactions with transcription factors. In contrast, the rapid and transient induction of miR-9, miR-146a and miR-155 has been speculated to negatively regulate the acute responses following activation of innate immune through down-regulation of proteins involved in the receptor-induced signalling pathways.