Active and less toxic vitamin D analogs could be useful for clinical applications. In the present study, we evaluated the toxicity and antitumor effect of two new synthetic analogs of vitamin D, namely PRI-1906 [(24E)-24a-Homo-(1S)-1,25-dihydroxyergocalciferol] and its side-chain unsaturated homo analog PRI-1907. The toxicity and calcemic activity, as well as antitumor effect of calcitriol analogs was investigated in vivo. The studies were performed in a mouse mammary 16/C cancer model. Since calcitriol and its analogs inhibited 16/C tumor growth only slightly, we applied them in the combined therapy with cyclophosphamide (CY). Moreover, cell cycle analysis and VDR and p27 expression were investigated. The LD50 values after five daily subcutaneous (s.c.) injections were 7.8, 10.0 and 2.4 microg/kg per day for calcitriol, PRI-1906 and PRI-1907, respectively. The serum calcium level increased to 40, 23 and 63% over the control for these compounds. We also compare the antitumor activity of the PRI-1906 with the calcitriol and previously studied PRI-2191 (1,24-dihydroxyvitamin D3, tacalcitol). Statistically significant inhibition of tumor growth by calcitriol up to the eighth day was observed in all schedules applied. PRI-1906 inhibited the tumor growth at doses 1 and 5 microg/kg per day, and PRI-2191 only at the dose 5 microg/kg per day. Addition of vitamin D analogs increased the antitumor effect of CY. PRI-1906 exhibited toxicity higher than PRI-2191 but lower than calcitriol and antitumor activity similar to both PRI-2191 and calcitriol. This new analog seems to be a good candidate for the combined treatment of mammary cancer.