The strain CN5364 of Escherichia coli which has an LD50 value below 106 organisms per mouse and is capable of forming slightly convex creamy white glistening colonies on blood agar, is cultivated in a Dorset-egg medium or a nutrient broth or agar medium of pH between 7.0 and 7.4, and at a temperature of about 37 DEG C.  The other cultural characteristics of the strain are disposed. The strain is inactivated or attenuated to produce a vaccine for the prevention or treatment of Escherichia coli infections.  A passively protective material may be obtained by immunising a susceptible warm-blodded animal with the above-mentioned strain and collecting serum from the animal.  The active strain exhibits the following LD50 values for mice: 103 intraperitoneally, 105 intravenously and 106 organisms subcutaneously. According to an example, a vaccine was made by growing the Escherichia coli strain CN 5364 on nutrient agar and washing the cells off in saline containing 0.25% formaldehyde.  The density was adjusted to a standard opacity and thiomersal was added to a concentration of 0.01%.  Mice, immunised by an intraperitoneal injection with 0.5 ml. of the formalized suspension, resisted both homologous and heterologous strains when challenged with them 4 days later.  In an alternative procedure heat at 60 DEG C. for 30 minutes or at 100 DEG C. for 10 minutes was employed for the inactivation instead of the formaldehyde.  In other examples a passively protective material was obtained by injecting horses intravenously, at intervals of 3-4 days, with increasing doses of 1, 2 and 3 ml. formalized suspensions of an inactivated Cn 5364 strain adjusted to a standard opacity.  The horses were then bled and the antisera was prepared and tested in mice.  A similar procedure was applied to calves instead of horses.ALSO:The strain CN 5364 of Escherichia coli, which which has an LD50 value below 106 organisms per mouse and is capable of forming slightly convex creamy white glistening colonies on blood agar, is inactivated or attenuated to produce a vaccine for the prevention or treatment of Escherichia coli infections.  A passively protective material may be obtained by immunising a susceptible warm-blooded animal with the above-mentioned strain and collecting serum from the animal.  The active strain exhibits the following LD50 values for mice: 103 intraperitoneally, 105 intravenously and 106 organisms subcutaneously.  According to an example, a vaccine was made by growing the Escherichia coli strain CN 5364 on nutrient agar and washing the cells off in saline containing 0.25% formaldehyde.  The density was adjusted to a standard opacity and thiomersal was added to a concentration of 0.01%. Mice, immunised by an intraperitoneal injection with 0.5 ml. of the formalised suspension, resisted both homologous and heterologous strains when challenged with them 4 days later.  In an alternative procedure heat at 60 DEG C for 30 minutes or at 100 DEG C for 10 minutes was employed for the inactivation instead of the formaldehyde.  In other examples a passively protective material was obtained by injecting horses intravenously, at intervals of 3-4 days, with increasing doses of 1, 2 and 3 ml. formalised suspensions of an inactivated CN 5364 strain adjusted to a standard opacity.  The horses were then bled and the antisera was prepared and tested in mice.  A similar procedure was applied to calves instead of horses.