Cytadsorption by Mycoplasma pneumoniae requires dense clustering of the surface membrane protein, P1, at the extreme end of the mycoplasma tip-like organelle. M. pneumoniae mutants incapable of cytadsorption either lack P1 or cannot mobilize and cluster P1 at the terminus. Specific cytadsorption-associated proteins in addition to P1 have been shown by mutant and revertant analysis to be essential for cytadsorption. Using monoclonal antibody probes and surface iodination techniques, additional chemical differences were observed between wild-type and mutant M. pneumoniae. M. genitalium, the recently identified new species, possesses structural and antigenic properties that appear similar to M. pneumoniae. Studies were initiated to establish the relatedness between M. pneumoniae and M. genitalium in terms of cytadsorption and membrane proteins.