Abstract Factors affecting the rates of plasmid transfer were investigated using Escherichia coli LC102 bearing a conjugative plasmid R100-1 and E. coli DH1. The rate constant of transconjugant increase, k ti, was used for presenting the degree of plasmid transmissibility instead of the plasmid transfer efficiency (pte). The rate constant was defined as the specific rate of transconjugant increase (srti, the number of transconjugants per donor per h) divided by the recipient cell concentration. The k ti values ranged between 10 −10 and 10 −15 ml cells −1 h −1, when estimated under various conditions. Moderate liquid agitation had a favorable effect on k tf but agitation rates higher than 33 s −1 (intergrated shear force) greatly decreased the value of k ti. The transconjugant-forming activity of the cells growing in continuous culture did not significantly change with the dilution rate, except those growing at dilution rates less than 0.1 h −1. The rate constant k ti at temperatures of 10–15°C was as low as the detection limit (10 −15 ml cells −1 h −1).