Significant amounts of immunoglobulins G and M, and a small amount of natural antibodies reactive with Neisseria gonorrhoeae and Escherichia coli, have been detected in human adult serums by imnmunofluorescent techniques. Umbilical cord serums also contained substantial immunoglobulin-G antibody but little or no M or A. These findings challenge the concept that natural antibodies reactive with Gram-negative bacteria are primarily of the immunoglobulin-M class.