The agglutination and consequent immobilization of actively motile cultures of Escherichia coli and Pseudomonas fluorescens by specific flagellar antisera may be reversed by treating them with proteolytic enzymes such as trypsin, pepsin, or bromelin. Reversion is not possible if more than 4 h elapse before it is attempted. Immune globulin fragments prevent immobilization when cultures are subsequently exposed to divalent sera; hence, the bacteria continue to be actively motile although their flagella are now "loaded" with globulin fragments. Distinct behavior differences are noted both when monotrichously flagellated (Pseudomonas) and peritrichously flagellated (Escherichia) bacteria are immobilized, as well as when such immobilization is reversed.