Accumulation of heat shock (stress) protein-70 (HSP-70) was assessed in corpora lutea of sheep obtained after in vivo administration of a luteolytic dose of prostaglandin (PG) F 2α. A quantitative immunofluorescence technique was used to localize inducible HSP-70 production to specific cell-types. The number and intensity of immunostained large luteal cells increased within 2 h of injection of PGF 2α. A drop in luteal progesterone concentrations (functional regression) was not manifested until 4 h post-treatment. A dramatic increase in intensely-stained mononuclear leukocytes was observed in luteal tissues at 16 h, when glandular weights had begun to diminish (structural regression). Stress proteins could mediate intracellular protein processing and cell-surface autoimmune mechanisms underlying luteal regression.