First-calf heifers were fitted with five different intramammary device models prior to, or within 2 mo after parturition. Each model represented an increase in the weight of the device to be supported by the epithelial cells of the gland cistern. Each model was tested in four animals by placing devices in two quarters while remaining quarters served as controls. Foremilk and stripping quarter secretion samples were collected prior to device placement and weekly thereafter and analyzed for total and differential cell counts, bacteriologic status, and NAGase activity. During the prebacterial challenge treatment period, foremilk and stripping SCC as well as percentage of neutrophils and NAGase activity in most device-fitted quarters remained significantly elevated over controls. Frequency of naturally occurring intramammary infection during the prechallenge period was lowest in quarters fitted with Models 2, 4, and 5. Frequency of infection with coagulase-negative staphylococci also tended to be lower in quarters fitted with Models 2, 4, and 5. Approximately 4 mo after device placement, all quarters were challenged by intracisternal inoculation with about 565 cfu Streptococcus uberis. Results across all models combined showed that although mean foremilk and striping SCC and percentage of neutrophils were significantly elevated in device-fitted quarters immediately prior to bacterial challenge, only Model 4 showed a protective effect. Several models were superior to others in elevating SCC and percentage of neutrophils, but no differences in susceptibility to Strep. uberis infection were observed among models based on these cytological characteristics.