The differentiation and activation of monocytes (MO) and monocytic cells is modulated by 1alpha,25-dihydroxyvitamin D3 (Vitamin D3). In order to investigate early effects on the differentiation process of MO, we used the mRNA Differential Display technology to identify genes that are induced in freshly isolated human blood MO cultured for 4 hours with Vitamin D3. A cDNA fragment was isolated and Northern analysis confirmed a low expression of this cDNA at about 1,4 kb in MO which was increased by the addition of Vitamin D3. Using the rapid amplification of cDNA Ends (RACE)-PCR we got a transcript (DDVit 1) of a length of 1251 bp containing an open reading frame that encodes a putative 16,5 kD protein. Database search revealed an identity with a possible enterocyte differentiation promoting factor with a length of 1177 bp that has not been further characterized. Therefore DDVit 1 may be a differentiation promoting factor for the monocytic lineage. Further investigations will clarify the role of this protein in the differentiation process of MO.