The cytotoxicity of synthetic amorphous silica particulates on mouse peritoneal macrophages in vitro was examined in an attempt to assess their fibrogenic potential in vivo. Fumed silicas and micronised silica gels were found to be as cytotoxic as a standard crystalline silica (Dorentrup No. 12); a precipitated silica was less cytotoxic. These results suggest that synthetic amorphous silicas would be pathogenic in vivo (after inhalation or intraperitoneal administration). However, available animal studies on other similar silicas indicate that they are only pathogenic after prolonged exposure. It is suggested that more in vivo information regarding the fibrogenicity of amorphous silicas is required.