effect of transfection manipulations on mouse cell cycle progression balbc3t3 mouse fibroblasts and a temperaturesensitive derivative ts 2e were transfected by the calcium phosphatedimethyl sulphoxide procedure to examine the effect of this manipulation on cell cycle progression cells were synchronized by growth to confluence in the presence of 214cthymidine to generally label cellular dna and then subcultured from the g0 state plasmid psv3neo or psv2neo dna was added to cells at 24 h postplating at peak s phase at designated intervals prior to during and after the transfection procedure cells were labelled with methyl3hthymidine for 1 h to monitor nascent dna synthesis and thereby assess cell cycle position in all experiments performed irrespective of the time of dna addition the transfection manipulations resulted in a reproducible transient interruption of cell cycle progression of about 5 h and manifested as a delay in movement across the subsequent g1s interface thereafter the cycle resumed normally the results indicated that the temporal sequence of the cell duplication cycle is altered when cells are exposed to exogenous dnaca3 po42