1. The effect of the novel, naturally occurring nucleotide 3'-5' cyclic diguanylic acid (c-di-GMP) on the lymphoblastoid Molt 4 cell line was studied. When exposed to this guanine nucleotide. Molt 4 cells exhibited a marked increase in [3H]thymidine incorporation, up to 200-fold at 50 microM c-di-GMP. Correspondingly, the DNA content of the treated cells was 9-fold higher than untreated cells. Stimulation of [3H]thymidine incorporation into the cells was time- and concentration-dependent. This effect was specific and was not observed with GMP or cyclic GMP, nor with the unhydrolysable GTP analogues, guanosine 5'-[gamma-thio]triphosphate and guanosine 5'-[beta gamma-imido]-triphosphate. C-di-GMP entrance into the cells was experimentally verified and occurred without using any means of cell permeabilization. SDS/PAGE analysis of cells exposed to [32P]c-di-GMP, followed by autoradiography, revealed the labelling of three low-molecular-mass proteins at 18-27 kDa. The labelling is highly specific to c-di-GMP and its extent was not affected by other guanine nucleotides. 2. One of the c-di-GMP-binding proteins was found to be the p21ras protein, by immunoprecipitation with the anti-Ras monoclonal antibody Y13-259. The effects described appear to be unique for c-di-GMP and, taken together, raise the possibility that an irreversible binding of this guanine nucleotide to the growth-promoting p21ras protein results in a fixed active conformation of this protein affecting DNA synthesis. Strikingly, although at 48 h of growth markedly high DNA levels were found in Molt 4 cells treated with c-di-GMP, this guanine nucleotide had no effect on cell replication during this period. Thus Molt 4 cells exposed to c-di-GMP enter the S phase uncoordinated with their overall replication rate.