MicroRNAs are key regulators of many biological processes, including cell differentiation. Here we show that during human monocyte-macrophage differentiation, expression of the microRNAs miR-223, miR-15a and miR-16 decreased considerably, which led to higher expression of the serine-threonine kinase IKKalpha in macrophages. In macrophages, higher IKKalpha expression in conjunction with stabilization of the kinase NIK induced larger amounts of p52. Because of low expression of the transcription factor RelB in untreated macrophages, high p52 expression repressed basal transcription of both canonical and noncanonical NF-kappaB target genes. However, proinflammatory stimuli in macrophages resulted in greater induction of noncanonical NF-kappaB target genes. Thus, a decrease in certain microRNAs probably prevents macrophage hyperactivation yet primes the macrophage for certain responses to proinflammatory stimuli.