Myeloid calcium binding proteins MRP-8 and MRP-14 were induced, and their genes were coordinately expressed, during differentiation of human leukemia HL-60 cells into macrophage-like cells after treatment with 1,25-dihydroxyvitamin D3 (VD3). Both MRP-8 and MRP-14 mRNAs appeared on the day after VD3 treatment. Their level reached a peak on day 2, and then quickly declined. Nuclear factors that interact with the 5'-upstream regions of MRP-8 and MRP-14 genes were studied with gel mobility-shift assays. Two factors (MP8FI and MP8FII) that interacted with 379 bp (426–48 bp upstream from the transcription-initiation site of MRP-8 gene) and 67 bp (-47 – +20) DNA fragments, respectively, were found in the cells treated with VD3 for 1 day. MP8FI and MP8FII were present neither in the nuclei of untreated HL-60 cells, nor in the nuclei of the cells treated with VD3 for 6 days. Human monocytic leukemia THP-1 cells, which constitutively expressed MRP genes, had MP8FII but not MF8FI. MP8FII was found to interact with the 19-mer sequence located just upstream of the TATA box. Also, two factors that bound to the different upstream regions (-400 – -150 and -149 – +50) of MRP-14 gene were detected in the differentiated HL-60 cells. One of these, MP14FI, appeared on day 1, but on day 6 its concentration greatly decreased. The other, MP14FII, was found in greater quantity on day 6 than on day 1. MP14FI, but not MP14FII, was found in THP-1 cells. These factors may be involved in the expression of MRP-8 and MRP-14 genes in VD3-differentiated HL-60 cells.