Using the method of Henkel and Taylor (11, 18) 60 Escherichia coli K 12 J 53 strains carrying R-plasmids of 15 incompatibility groups (FI, FIme, FII, FIV, FV, FVI, I alpha, I gamma, I omega, I5, B, C, K, M, S) have been tested with respect to their serum sensitivity in pooled human serum. We found for 41 plasmid carrying strains a grade of resistance to serum bactericidal activity of 0, for 19 strains a grade of 1, 2 or 3 according to the method of Henkel. Applying the method the method of Taylor the results of the bactericidal tests of the same 60 strains could all graded in category 1. The results obtained show that there was not any change of sensitivity to serum bactericidal activity after the uptake of a plasmid. 29 strains carrying different plasmids of the same 15 incompatibility groups were selected for studies in a low concentration system of serum. Survivors' curves in 1, 2, 3, 5, 10 and 20% of serum after different times of incubation (10, 20, 30, 60, 90, 120 and 180 min) were established. In relation to the control strain E. coli K 12 J 53 R- a difference in the percentage of survivors of 18 strains carrying plasmids of 10 incompatibility groups (FI, FIme, FIV, FVI, I alpha, I omega, I5, C, K, and M) could be demonstrated. However, this could only be observed in concentrations of serum from 1 to 5% and most clearly after an incubation time of 10 min. The results were graded into 3 categories in dependence of the difference of the percentage of survivors to the control strain. The plasmid pIE 307 and pIE 403 (Inc FI and C) showed the strongest activity in increasing the serum resistance. We found the method used to be suitable for the estimation of small alterations of serum sensitivity. The results obtained in E. coli K 12 made us control their influence in other E. coli strains carrying a complete O-antigen.