c‐Myc induces widely varying cellular effects, including cell proliferation and cell death. These different cellular effects are determined, in part, by c‐Myc protein expression levels, which are regulated through several transcriptional and post‐transcriptional pathways. c‐Myc transcripts can be detected in cells at all stages of B and T lymphocyte development. However, little is known about c‐Myc protein expression, and how it varies, in developing lymphocytes. Here mice have been generated in which the endogenous c‐Myc locus has been modified (c‐Myc^G) so that it encodes a GFP‐c‐Myc fusion protein. c‐Myc^G/G mice are viable, appear normal and exhibit grossly normal lymphocyte development. Flow cytometric analyses revealed significant heterogeneity in c‐Myc protein expression levels in developing c‐Myc^G/G B and T lymphocytes. GFP‐c‐Myc expression levels were highest in proliferating lymphocytes, suggesting that c‐Myc up‐regulation is important for promoting lymphocyte cell division, and demonstrating that GFP‐c‐Myc expression is a marker of proliferating lymphocytes in vivo.