Secretion of thymidine appeared to be a common property of hybridoma and myeloma cells, but not of other cell types, which were tested. Of three hybridoma cell lines tested, all secreted thymidine in amounts resulting in the accumulation of thymidine to concentrations of 10-20 mu M in the culture medium. Also three of five myeloma cell lines that were analyzed secrete thymidine, but none of the other cell types that were studied. Thymidine was purified to homogeneity (4 mg purified from 31 of culture medium) and identified as such by nuclear magnetic resonance spectroscopy. The cells that secreted thymidine showed high resistance to the growth inhibitory effect of thymidine. (c) 2006 Elsevier Inc. All rights reserved.