<jats:p> Cell-wall preparations were made from more than 140 strains of aerobic actinomycetes representing most of the form-genera that have been proposed. All cell-wall preparations contained as major constituents glucosamine, muramic acid, alanine, and glutamic acid. In addition, cell-wall preparations from various types of streptomycetes and strains of <jats:italic>Microëllobosporia</jats:italic> contained glycine and <jats:sc>ll</jats:sc> -α,ε-diaminopimelic acid; those from strains of most Actinoplanaceae and micromonosporae contained glycine and meso-α-ε-diaminopimelic acid; those from strains of <jats:italic>Thermoactinomyces, Microbispora, Dermatophilus</jats:italic> , and nocardiae of the <jats:italic>madurae-pelletieri</jats:italic> group contained meso-α,ε-diaminopimelic acid; and those from strains of <jats:italic>Thermomonospora, Micropolyspora</jats:italic> , and most nocardiae contained meso-α,ε-diaminopimelic acid, arabinose, and galactose. All the strains used were also studied morphologically.