The effect of 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) and dichloromethylidene-bisphosphonate (Cl2MBP) on the structure of the organic matrix of heterotopically induced bone in guinea pig was studied. Heterotopic bone formation was induced by transplantation of allogenic urinary bladder epithelium. Starting from the day of transplantation the animals were treated subcutaneously with HEBP and Cl2MBP with a dose of 12.5 mg P/kg/day during 35 days. The control group was injected with 0.9% NaCl solution. The advantage of heterotopic bone induction as an experimental model is the fact that the applied drugs act on de novo bone formation. Collagen fibers were treated as markers of bone because their size and spatial arrangement reflect the structure and maturity of organic matrix of this tissue. Decalcified histological sections of induced bone, taken 35 days after implantation of inductor, were stained by the picrosirius method. This staining enhances the natural birefringency of collagen fibers and allows for better and specific visualization of collagen fibers bundles under polarizing microscope. In this way the amount of information in the analysed image is increased. Thirty five microphotographs were analysed from each of the investigated groups with the use of optical diffractometry. The radial distribution of light intensity in diffraction patterns was analysed what allowed to evaluate spatial frequencies connected with the width of collagen bundles in induced bone tissue. Since the spatial arrangement of collagen fibers in newly formed bone is random, analysis of angular distribution of light intensity in diffractograms was not performed. Using discriminant analysis the significant differences between all three studied groups of animals were found.(ABSTRACT TRUNCATED AT 250 WORDS)