Abstract
Colchicine injected 5, 9 and 24 h respectively before initiation (using s.c. injection of urethane for initiating action and TPA skin applications for promoting action, in female ICR mice) led to a significant increase in skin tumour incidence in the --9-h group, and an increase in percentage malignancy in both the --5- and --9-h groups. These times corresponded to the peak of metaphase arrest by the colchicine. The results are discussed in relation in those of Pound and Withers (1963) and others, who found that mitotic stimulation at the time of urethane initiating action raised the ultimate tumour incidence; and the inference is drawn that initiating action in mouse skin may occur during the M phase, rather than during the G1, S, or G2 phases, as suggested by others.