In the analysis of plasmid profile obtained by agarose gel electrophoresis, the strain harvoring multiple reference plasmids of known molecular weight were needed to estimate the size of unknown molecular weight plasmids. Six strains of E. coli isolated from clinical specimens carried multiple plasmids and these strains could be available as a reference plasmids harvoring strain. These E. coli strains showed 4 to 9 plasmids of various size ranging 2.5 to 94.3 megadalton (Mdal). The correlation coefficients of linear regression between the relative mobility and molecular weight were 0.99966 to 1.00000. Among them, E. coli KE327 from throat which contained 7 plasm ids as follows: 79 Mdal, 46 Mdal, 33 Mdal(pKY3027 C), 4.9Mdal, 3.8Mdal(pKY3027 E), 3.5Mdal, and 2.7 Mdal. Relative amount of the pKY3027 C was the smallest(7.09%) and that of the pKY3027 E was the largest (24.24%) among the plasmid fractions of E. coli KE327.