Cell wall antigens from each of six Cellulomonas species and two soil isolates were prepared. Cross-reactions were set up with antisera obtained through inoculation of New Zealand white rabbits. The percentage of cross-reaction ranged from 12 to 93. The known cellulomonads clustered in a range of 68 to 93% with one exception: C. uda with C. subalbus at 43%. The soil isolates demonstrated a much lower affinity and thus were hot considered cellulomonads. The cellulomonads, although similar to each other, demonstrated enough diversification to question their reduction to a single species.