The host component of control of scrapie incubation period in the mouse is manifested largely through the action of the Sinc gene. Only one mouse strain (VM) has been found that is p7p7 (prolonged incubation for ME7 agent) and two other strains have been derived from VM. All other strains, designated s7s7, have a short incubation for ME7. In the present study, the I strain was shown to fulfil the criteria that are characteristic of mouse strains with the p7 allele of Sinc: a comparatively long incubation period for ME7 and a short incubation period for 22A, the incubation period for F1 hybrid mice (s7s7 X p7p7) either fell between the incubation periods for the parental strains (with ME7) or were longer than either parent (with 139A and 22A), amyloid plaques occurred following injection of ME7 and 87V but not after 22A or 139A, lesion profiles for four scrapie strains were similar in I mice and p7p7 mouse strains, and injection of 87V led to disease in less than 300 days. Finally, allelism tests using F1 hybrid mice (I X a p7p7 mouse strain) and progeny of backcrosses between these F1 mice and I mice failed to reveal the segregation of additional major genes affecting scrapie incubation period.