Cellular retinoic acid binding proteins (CRABP) are low molecular weight proteins whose precise function remains unknown. They bind retinoids and may thereby modulate the intracellular steady-state concentration of retinoids. Whereas CRABP I is ubiquitously expressed, CRABP II is mainly detected in various cell types of the skin. By representative difference analysis we found that CRABP II is also strongly expressed in human monocyte-derived macrophages (MAC) but not in freshly isolated monocytes (MO). The CRABP II mRNA was gradually upregulated during differentiation from MO to MAC in the presence of 2% serum. Adherence, which is important for MO differentiation, induced CRABP II expression, but the addition of 10(-7) M retinoic acid inhibited the upregulation of CRABP II expression during MO/MAC differentiation. As MO can differentiate along the classical pathway not only to MAC but also to dendritic cells we analyzed the expression of CRABP II in MO-derived dendritic cells cultured with 10% FCS, IL-4, and GM-CSF. In contrast to MAC, MO-derived dendritic cells showed an extremely low expression of CRABP II. From these results we conclude (1) that the availability and the metabolism of retinoids may be different in MAC compared to MO and dendritic cells and (2) that this may influence differentiation and activation of those cells.