Cytospins of the MCF-7 cell line and 93 consecutive smears of human malignant neoplasms were immunochemically evaluated for the expression of proliferating cell nuclear antigen (PCNA) and Ki-67-related antigen. Results expressed as the labeling index (LI) were compared with histologic sections. The PCNA LI and Ki-67 LI were lower in cytologic smears than in histologic sections, though the differences were not statistically significant. A positive linear relationship was found between these markers in both cytologic and histologic samples. The PCNA LI was generally lower than the Ki-67 LI, but in seven malignant neoplasms, PCNA LI was greater than the corresponding values of the KI-67 LI. We conclude that cell proliferation can be reliably evaluated on cytologic preparations; PCNA may behave as a Ki-67-like reagent in some tumors, and PCNA may sometimes overestimate the cell growth fraction assessed by Ki-67 immunoreactivity.