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The in vivo study of cell kinetics of squamous carcinomas of the head and neck is in its infancy. One cell kinetic parameter, the labelling index (LI), the percentage of cells actively replicating DNA, was estimated in carcinomas of the oral cavity (10), larynx (6) and pharynx(5) by an histochemical immunoglobulin technique developed recently in our laboratory: the antinucleoside antibody technique (ANIP-LI). Data from earlier experiments using autoradiography (3H-thymidine) have established that the enzyme-antibody and isotopic techniques are equivalent. The LI ranged from 1 to 30 with a mean of 13 in this group of tumors. The LI could not be evaluated for 14% (3/21) of the specimens. The 3 patients with tumors not suitable for ANIP-LI had received a full course of radiotherapy and/or chemotherapy before they had entered our study. The higher the LI of the tumor, the greater the likelihood that the surgical stage would be higher than the clinical stage, most often as a result of clinically unrecognized lymph node metastasis. However, the number of tumors sampled was inadequate for a statistical trend to be identified. The potential for the LI to be useful in the formulation of improved therapeutic strategies awaits the accumulation of more data from studies currently in progress.