The p68 protein is a highly conserved nuclear antigen that is thought to be important in the regulation of cell growth and division. It is found in dividing cells of all mammals and amphibians tested, but not in quiescent cells. The protein shows a distinct granular distribution in the nucleus and is induced within four hours of serum stimulation of quiescent mouse fibroblasts. The p68 protein was first identified because of its specific immunological cross-reaction with the DNA tumour virus nuclear oncogene SV40 large T, detected with the anti-SV40 large T monoclonal antibody DL3C4, now renamed PAb204. Sequencing of human complementary DNA coding for the growth-regulated p68 nuclear protein has revealed the molecular basis for its cross-reaction with SV40 large T antigen and its extensive homology with the translation initiation factor eIF-4A. The sequence similarity between p68 and eIF-4A is interesting because eIF-4A acts as an ATP-dependent RNA helicase and T antigen is an ATP-dependent DNA helicase. We suggest that p68 could be a DNA or RNA helicase in the cell nucleus which is involved in replication, transcription or RNA processing and is required for cell growth.