The effects of pre- and posttransplant immunization on graft survival, infiltrate intensity, and host in situ/systemic cellular immune responsiveness were examined for class I MHC-disparate rat renal allograft recipients. Naive, unsensitized PVG (RT1c) recipients of class I MHC disparate PVG.R1 (RT1.Aa on PVG background) orthotopic kidney transplants displayed long-term (greater than 50 days) survival (LTS) in a majority (41/52) of cases. Pretransplant immunization of recipients with a PVG.R1 skin graft most often resulted in rejection (mean survival greater than 21.3 days) with 8/15 rats surviving less than or equal to 2 weeks and only 3/15 with LTS. Pretransplant immunization with a skin graft from a fully MHC-disparate PVG.1A (RT1a on PVG background) donor resulted in acute rejection (mean = 6.1 days) with 0/8 rats surviving greater than or equal to 2 weeks. Donor-specific class I and II disparate (PVG.1A), and third-party (LEW) skin transplants applied on LTS (greater than 50 days) PVG.R1 kidney graft recipients showed typical 1st- and accelerated 2nd-set skin graft rejection, but had no effect on kidney graft survival. In contrast, 6/7 LTS PVG.R1 kidney graft recipients accepted PVG.R1 skin grafts indefinitely following their transient partial rejection. Histologic analysis of kidney allografts revealed the highest degree of mononuclear cell infiltrates in animals specifically sensitized by PVG.1A skin grafts prior to transplant. Donor class I-specific cytotoxic T lymphocyte precursor (pCTL) frequencies, as determined by limiting dilution assays, were increased and equivalent at 1 week posttransplant in kidney allograft cell eluates from nonrejecting naive recipients (1/127-1/2209) and rejecting presensitized animals (1/470-1/7848). LTS animals had decreased intragraft pCTL at greater than 50 days (1/2969-1/61875), as did LTS at greater than 50 days that received PVG.R1, PVG.1A, or LEW skin grafts posttransplant. In all groups, splenocyte pCTL frequencies were significantly lower than the corresponding values within the allograft. By comparison, no significant differences in intragraft or splenic proliferative T lymphocyte (pPTL) precursor frequencies were observed between any groups. These results indicate that unsensitized recipients of class I-disparate renal cells grafts are capable of maintaining graft survival in the early posttransplant period, despite the presence of significant in situ antidonor class I MHC-specific cellular immune responsiveness. These findings also indicate that long-surviving PVG recipients of class I-disparate renal allografts develop specific functional tolerance to donor class I alloantigens, that may be associated with a diminished frequency of anti-class I cytotoxic (but not proliferative) T cell precursors.