The vitamin D metabolite 1,25(OH)2D3 has long been known to inhibit growth of prostate cancer cells and this has been attributed to a VDR-mediated pathway controlling target gene expression, resulting in cell cycle arrest, apoptosis and differentiation. New research has shown that another vitamin D metabolite, 24,25(OH)2D3, inhibits proliferation of prostate cancer cells as well, more specifically, cells of the line LNCaP. It is not clear exactly how 24,25(OH)2D3 exerts this cancer growth inhibition but it has been shown that it is to some extent regulated via G protein coupled signalling pathways. Catalase is a haem-containing redox enzyme found in the majority of animal cells, plant cells and aerobic microorganisms. This enzyme is very important because it prevents excessive accumulation of the strongly oxidizing agent H2O2 which otherwise can do damage to the cells. Because of this preventive effect of catalase, important cellular processes which generate H2O2 as by-product can proceed safely. Biochemical analysis of catalase has shown that it binds endogenously to 24,25(OH)2D3. The fact that 24,25(OH)2D3 has anti-proliferative effects on prostate cancer cells combined with the fact that it binds to catalase generates the hypothesis that this binding interferes with the essential task of catalase to keep the cell free from accumulation of destructive H2O2, and by means of this interference induces apoptosis. Finding out about the cancer growth inhibiting mechanism behind each vitamin D metabolite is important and may be a lead in the search for a new, better treatment of prostate cancer. The specific aim of this project was to study if and in what way 24,25(OH)2D3 affects the enzymatic activity of catalase in LNCaP cells and to do this with dose and time responses in focus. In this experiment LNCaP cells were incubated for 48 hours together with 24,25(OH)2D3 in five different concentrations, then a catalase assay was performed on the cells including fluorescence-mediated measuring of catalase activity in both treated and untreated cells. The analysis of the result values showed that regardless of dose or time, 24,25(OH)2D3 has no statistically significant effect on catalase activity in cells of the line LNCaP.