Immunofluorescence studies using PCNA autoantibodies specific for the proliferation-sensitive protein cyclin have revealed dramatic changes in the nuclear distribution of this protein during the S-phase of normal and transformed cells. Patterns of cyclin antigen distribution subdivide S-phase and have provided new cell cycle landmarks. Some of these (nucleolar exclusion or staining), mimic topographical patterns of DNA synthesis thus arguing for a role of this protein in some specific aspect of DNA replication. Cells outside S-phase (G0 included) stain only weakly with PCNA antibodies, stressing the usefulness of this reagent for identifying proliferating cells (S-phase cells) of both normal and malignant origins.