Autosomal genes that are subject to random allelic inactivation (RAI), like imprinted genes [1] and genes subject to X-inactivation [2], require mechanisms that dictate the differential transcriptional regulation of two sequence-identical alleles. RAI genes include olfactory receptor genes [3], and the various genes encoding antigen receptors on lymphocytes (immunoglobulin genes, T cell receptor genes and NK receptor genes [4–7]). These observations raise the possibility that other genes might be similarly regulated. Moreover, an interesting possibility is that certain genes might be monoallelically expressed in some cells and biallelically expressed in others. Recently, reports of monoallelic expression of interleukin-2 (IL-2) [8,9] and IL-4 [10,11] have raised the possibility that the cytokine gene family may be subject to monoallelic expression. Another report suggests that the gene encoding the transcription factor Pax-5, which is involved in B-cell (and cerebellar) development [12,13], is also subject to monoallelic expression [14]. Using a novel single-cell reverse transcription-polymerase chain reaction (RT-PCR) approach, we have analyzed the IL-2 and Pax-5 genes in mice. We found that IL-2 is monoallelically transcribed in some T cells and biallelically transcribed in others, raising interesting questions regarding cytokine gene regulation. Additionally, our analyses suggest that Pax-5 is consistently biallelically transcribed. Thus, the IL-2 gene and other cytokine genes may be regulated in a stochastic manner that results in 0, 1 or 2 alleles of a given cytokine gene expressed in each T cell. This type of regulation could account for the wide variety of different combinations of cytokine genes expressed in individual T cells and therefore plays a role in the generation of T cells with a range of different effector functions.