Metallocene dihalides, which are cyclopentadienyl complexes with the general formula R2MX2 (where R=eta(5)-C5H5, eta(5)-CH3C5H4, eta(5)-SiMe3C5H4 etc.; M=Ti, Zr, Hf, V or Nb; and X=halogen), are highly effective agents against Ehrlich ascites tumour cells and lymphocytic leukaemia. The aim of this study was to evaluate the antitumor activity of the various metallocene dihalides and particularly their effects on cell proliferation of human breast and colon cancer cells. The growth inhibition of the antitumour metallocenes (eta(5)-C5H5)2TiCl2 and (eta(5)-C5H5)2VCl2 and four ring-substituted derivatives in HT-29 (colon cancer) and MCF-7 (breast cancer) cell lines is reported. The results showed that ring-substitution of metallocenes gave similar or even better activity in cell proliferation reduction, in both cell lines, especially in HT-29 and suggested that ring-substitution may enhance the inhibitory activity of the metallocene compound family.