ABSTRACT: Sialic acid was quantified in plasma membranes of corpora lutea isolated during prostaglandin (PG) F2α-induced luteolysis in sheep. Concentrations of sialic acid within membranes decreased after injection of PGF2α, and before signs of luteal regression (i.e., a decline in tissue concentrations of progesterone) were manifested. Removal of residues of sialic acid from luteal membranes was not associated with cellular binding of gamma globulin, as monitored by indirect immunofluorescence microscopy. We suggest that desialylation of luteal membranes could be an important aspect of the mechanism of luteolysis. Such a process does not appear to involve participation of autoantibody.