The expression of p34cdc2 has been examined in normal, hyperplastic, and malignant oral epithelia and lymphoid tissues. Four monoclonal antibodies were prepared against Xenopus p34cdc2, three of which react specifically with human p34cdc2 and not with p33cdk2. These produced similar patterns of staining in both fixed and frozen sections of human material. Staining occurred mainly in the proliferative compartments of normal and hyperplastic tissues. In normal oral epithelia, parabasal and basal cells were the most strongly stained, with lighter cytoplasmic staining in lower prickle cells. In tonsillar germinal centres, a high proportion of cells was stained, with fewer positive cells in interfollicular zones, a distribution in keeping with the known pattern of cell proliferation. In normal cells, the intracellular location of p34cdc2 was cytoplasmic until early prophase, but in oral squamous cell carcinomas and lymphomas, it was located in both cytoplasm and nucleus during interphase and a larger fraction of cells was positive than in the equivalent normal tissues. Higher-grade neoplasms showed both a higher intensity of staining and a higher proportion of p34cdc2-positive cells.