PMCA in the presence of Mabs was also used to study the importance of binding site specificity in the PrP C to PrP Sc conversion process (Fig. 3) . We performed sPMCA with different Mab concentrations to determine the minimum amount of Mab necessary to inhibit the conversion process. Using a 10 24 dilution of 263K-infected hamster brain homogenate as the PrP Sc seed and a 10% normal brain homogenate (NBH) as the source of PrP C , we tested the ability of Mabs to inhibit the conversion of PrP C to PrP Sc . For each Mab, final concentrations of 12 mg/ml (lanes 3, 6, 9, 12, 15, 18, 21 and 24), 25 mg/ml (lanes 4, 7, 10, 13, 16, 19, 22 and 25) and 50 mg/ml (lanes 5, 8, 11, 14, 17, 20, 23, and 26) were prepared in hamster NBH and used in the sPMCA reactions. Compared to sPMCA 40 , which contained no Mab (lane 1) and with the exception of 02-3/3A2, the majority of the PrP-specific Mabs inhibited the conversion process in a dose-related manner although some were more effective than others. Mabs 7E4, 10E4, 11F12, 8E11, and 8E9 completely inhibited the conversion process at 50 mg/ml while Mabs 1B11 and 2F7 inhibited the conversion process to a lesser degree. The inhibition caused by the Mabs was a specific response since sPMCA 40 studies replacing Mabs with purified normal mouse IgG (at 12-50 mg/ml) in the 10% NBH did not cause any inhibition of PrP Sc formation (data not shown). It is interesting to note that, with the exception of only 8E9, the epitopes for all the Mabs that caused complete inhibition are located in the amino half of the PrP while those that caused incomplete inhibition are located in the carboxy half of PrP. There was good correlation between the extent of PrP Sc inhibition when 10 mg/ml Mab in cell culture was compared to 12 mg/ml Mab with sPMCA 40 .