The effects of freshly ground silica (14808607) particles and aged silica dust were compared in-vivo to determine if the inhalation of freshly cleaved silica was more toxic and inflammatory than exposure to aged silica of the same composition and particle size. Male Fischer-344-rats were exposed to filtered air (control), freshly milled silica, or aged silica. Silica exposed animals inhaled concentrations of 20mg/m3 silica in air for 5 hours/day, 5 days/week for 2 weeks; pulmonary responses were determined 1 to 3 days after exposure. Following exposure, animals were sacrificed, and lungs and trachea were weighed and lavaged. The measurement of surface radials via electron spin resonance spectroscopy indicated that freshly milled quartz particles displayed 54% more silicon based free radicals than aged silica. Aged silica exposed rats displayed significantly increased levels of total cells, red blood cells, and leukocytes harvested via bronchoalveolar lavage. Freshly milled quartz exposed animals displayed significantly more alveolar damage and inflammation compared to animals exposed to aged silica. Acellular levels of both protein and phospholipid in the lavage were significantly increased after the inhalation of aged silica. The authors conclude that the inhalation of freshly cleaved silica causes significantly greater cytotoxicity and inflammation compared to equal concentrations of aged silica.