The polypeptide ubiquitin covalently binds to cytoplasmic proteins and marks them for proteolytic degradation. Ubiquitin is upregulated during apoptosis in some systems. Apoptosis increases during luteolysis but it is not known whether ubiquitin is expressed in regressing corpora lutea. Marmoset ovaries were removed on day 10 of the luteal phase from animals that had received either no treatment, treatment with the PGF(2α) analogue cloprostenol 24 h earlier, or treatment with the GnRH antagonist antarelix for either 24 or 48 h before ovary collection. Ubiquitin was localized on ovarian sections by immunocytochemistry, and oligonucleosome formation characteristic of apoptosis was examined in isolated corpora lutea by electrophoresis of extracted [32P]DNA. Oligonucleosome formation was low in midluteal corpora lutea on day 10 but increased after induced luteal regression with PGF(2α) and GnRH antagonist. Nuclear ubiquitin immunoreactivity was found in 1.66 ± 0.66 steroidogenic cells and cytoplasmic staining was found in 0.4 ± 0.3 steroidogenic cells (per x 40 field of view) in midluteal phase corpora lutea on day 10. Luteolytic induction with PGF(2α) significantly increased the number of cells exhibiting cytoplasmic immunoreactivity to 12.24 ± 1.6 (P < 0.05). Ubiquitin immunoreactivity was not observed after GnRH-induced luteal regression. Apoptotic oligonucleosome formation was found after induced luteal regression with both PGF(2α) and GnRH antagonist, but ubiquitin upregulation only occurred after PGF(2α)-induced regression. These results indicate that ubiquitin expression is not specific for luteolysis and is not an indicator of luteal apoptosis, but that the polypeptide does play a role in luteal cellular responses to PGF(2α).