The physical and chemical characteristics of synthesized ultrafine magnetic minerals, ferrite particles, and their suppressive effect on the growth of turfgrass mold were studied. These ferrite particles might be applicable to environmental control, for example, to reduce the amount of agricultural chemicals used. Several kinds of ultrafine ferrite panicles, such as magnetite Fe3O4, MnFe2O4, CaFe2O4 and ZnFe2O4 were prepared by a chemical coprecipitation method in an alkaline aqueous solution with a pH of 13 at 100 degreesC for 2 h. Their crystal structure was that of spinel according to X-ray diffraction measurements and the lattice constants were almost the same as those of bulk ferrites. The average crystallite size calculated using Scherrer's formula ranged from 5 to 30 nm, which was in accord with the values obtained by transmission electron microscopic observations. The particles of these ferrites were almost granular in shape and were well crystallized. The specific surface area of these ferrite particles ranged from about 90 to 230 m(2)/g. The values of saturation magnetization of these ferrite particles were found to be less than those of bulk ferrites, The coercive force, H-1(C), of these ferrite particles was related to the crystalline magnetic anisotropy constant of bulk ferrites. Turfgrass mold, Sclerotinia homoeocarpa, which is of worldwide importance was propagated in an incubator at temperatures ranging from 10 degreesC to 35 degreesC in total darkness. The medium was potato dextrose agar (PDA) and each ultrafine ferrite was uniformly dispersed throughout. The optimum growth temperature of the mold was 25 degreesC. MnFe2O4, CaFe2O4, Fe3O4 and ZnFe2O4 suppressed growth at 10 degreesC but did not so clearly at 25 degreesC. CoFe2O4 and Ni0.5Zn0.5Fe2O4 suppressed growth by more than 40% at both 10 degreesC and 25 degreesC. with CoFe2O4 having strongest suppressive effect of about 70%. The ultrafine ferrite particles with a high Ni2+ content in the NiZnFe2O4 system, such as Ni0.8Zn0.2Fe2O4. dramatically suppressed growth. The pH did not seem to be a factor involved in growth suppression in our experiments. The magnetic field and the influence of the chemically free ions by themselves can not explain the observed growth suppression.