PTHrP has been shown to be the primary factor responsible for humoral hypercalcemia of malignancy, and recently PTHrP has been shown to be an early-response gene that may be involved in cellular proliferation or differentiation. 1,25 DHCC and its analoge EB1089 have been reported to inhibits growth of cells derived from a variety of tumours in vitro and in vivo. We have investigated in this study the effect of different 1,25 DHCC and EB1089 doses (from 10−9 M to 10−7 M) at 24 hours on the PTHrP mRNA expression in prostate cancer PC3 cell line and breast cancer MCF7 cell line and the PTHrP mRNA levels were examined by Northern blot analysis. PTHrP mRNA levels were lower with EB1089 at than those in cells treated with 1,25 DHCC which means that the EB1089 was more effective than 1,25 DHCC on these cells. In addition, our data shows that the EB1089 was more effective in PC3 cells than in MCF7. We found that the 10−8 M was more effective concentration in preventing the induction of the PTHrP mRNA. We conclude our present results indicate that both 1,25 DHCC and EB1089 suppress PTHrP gene transcription in prostate cancer PC3 cell line and breast cancer MCF7 cell line, although the mechanism for this is unknown yet. Therefore, these compounds may provide a new strategy to treat malignancies, because of their potent ability to prevent the PTHrP-induced humoral hyper calcaemia of malignancy factor.