The transcriptional regulation of myeloperoxidase (MPO) and lactoferrin (LF) was examined during terminal myeloid differentiation of the murine cell line 32D C13. The rates of transcription initiation for MPO and LF, determined by an in vitro nuclear run-on assay, increased approximately ninefold. The accumulation of MPO mRNA in 32D C13 cells, determined by Northern blot analysis, correlated temporally with the observed increase in MPO transcription initiation. On the other hand, accumulation of LF mRNA lagged behind the observed increase in LF transcription initiation. In mouse L cells, the LF gene was transcribed more frequently than the MPO gene, though neither mRNA accumulated. Finally, murine MPO transcription is shown, by Northern blot and primer extension analysis, to initiate at multiple sites. These results indicate that whereas transcription induction may largely account for the accumulation of MPO mRNA during terminal myeloid differentiation, both transcriptional and posttranscriptional mechanisms operate to allow accumulation of LF mRNA. The 32D C13 cell system will be a useful model for elucidating these mechanisms.