The ultrastructural aspects of the interaction ofMycoplasma gallisepticumwith specific rabbit antibody have been studied. In particular, fixation conditions which allow the simultaneous preservation of cellular fine structure and membrane antigenicity have been established and applied in a procedure of indirect immunological labelling of the antibody‐coated organisms with ferritin conjugated sheep anti‐rabbit IgG. The advantages of working with agar embedded organisms in a multistep labelling procedure are discussed. In membrane fractions ofM. gallisepticum, prepared by osmolysis and freeze‐thawing, only sealed membranes retained their antibody‐binding capacity. Electron microscopical examination of “break‐through” colanies from immune growth inhibition zones revealed that the majority of cells in these colonies were destroyed, sometimes limited only by a single‐layered membrane and without extracellular antibody coat. An exception from this was the presumedly young cells in the periphery of colonies and in microcolonies which appeared to be intact and had a heavy antibody layer surrounding the cells. Based on these characteristics, a possible sequence of events is suggested eventually leading to destruction of mycoplasma organisms in immune growth i