Cyclins play an important role in regulating the passage of dividing cells through critical checkpoints in cell cycle. Aberrant exprssion of cylcin proteins has been found in a number of human cancers, including carcinomas of the head and neck, where amplification of the cyclin D1 gene is a common finding. The obective of this study was to examine cell cycle kinetics in oral carcinomas by determining the expression the S phase protein cylcin A and the M phase protein cyclin B1. Routinely processed tissue sections of 50 oral squamous cell carcinomas from the floor of the mouth were stained by immunohistochemistry for cyclin A, cyclin B1 and Ki-67 proteins. Ten specimens of normal epithelium from the floor of the mouth were used as controls. Teh number of cells showing nuclear staining for cylin A, cyclin B1 and Ki-67 proteins was determined by computer image analysis. There were 17 well-differentiated, 25 moderately differentiated and 8 poorly differentiated tumours. Mean counts for cylcin A (29.50 ± 4.10, Mean ± 95% CI), cyclin B1 (2.05 ± 0.30) and Ki-67 (49.46 ± 5.91) protiens in the carcinomas were significantly higher than counts for the normal epithelial controls (cyclin A: 930 ± 1.72; cyclin B1: 1.01 ± 0.36; Ki-67: 17.4. ± 4.17). For cyclin A, cyclin B1 and Ki-67, mean staining scores fosr all tumour grades were significantly higher than controsl. There was a strong correlation between ki-67 and cyclin A scores in all tumour groups (r2=0.68); however, the correlations between cyclin B1 and Cyclin A Scores(r2=0.35) and between clylcin B1 and ki-67 scores (r2=0.39) were weak. We conclude that there is overexpression of cyclin A and cyclin B1 proteins in oral carcinoma. Furthermore, the poor correlations for cylcin B1 scores with other cell cycle indices suggest that there may be aberrant cell cycle progression at G2/M checkpoint in oral carcinomas.