The promyelocytic human cell line U937, cultured in the presence of TPA and/or vit. D3, differentiates to monocytes and to macrophage-like cells. A potent stimulus for differentiation is represented also by colony stimulating factor-1 (CSF-1). Since this factor is a strong inducer of PGH synthase in human monocytes, we have investigated whether this event may be connected to the differentiation of U937. We have found that TPA, in the presence of serum, increased the production of thromboxane B2 (TXB2) 4-5 fold, while DMSO, which induced differentiation to neutrophils, was not active. Here we report studies indicating that the effect of protein and RNA synthesis inhibitors on prostanoid production, in cells incubated in the presence of CSF-1 (or FCS), can be correlated with an inductive event carried out by the growth factor, as demonstrated by the use of Western and Northern blotting procedures. However, while in human monocytes PGH-s and its mRNA are absent in controls and are expressed at high levels in CSF-1 stimulated cells, in U937 cells exposed to TPA, PGH-s mRNA was clearly detected by Northern blots, but its translation product was expressed at low level, and cells generated low amounts of TXA2 (13% of maximal production). After incubation with CSF-1 (or FCS) mRNA levels were only slightly modified, but large amounts of TXA2 accumulated in the medium. We have interpreted these findings by suggesting that CSF-1 is capable not only of regulating the expression of the gene encoding PGHs, but also of acing translationally to regulate the expression of its mature mRNA.