SUMMARYThe cultural and pathogenic properties of five rough mutants of avian E. coli were compared with those of four smooth strains. None of the smooth strains showed the ability to agglutinate in aqueous solutions of acriflavine higher than 1:500. On this basis they could be differentiated from the rough strains. In contrast to the smooth strains none of the rough strains grew on D1.5 medium containing 400 ig/ml of crystal violet as an inhibitor. Endotoxin potency was investigated by inoculating 10-day-old chick embryos with bacteria which were dried with alcohol, aceton and ether under vacuum in the presence of CaCl 2 . Pathogenicity of live bacteria was tested by inoculation of 1-day-old commercial broiler chicks via their yolk sacs. The virulence of two live rough mutants (R1 and R2) in a concentration of 10 4 colony forming units (CFU)/ml of inoculum proved to be higher than that of the smooth strains. Live bacteria of both smooth and rough E. coli under experimental conditions produced the same gross pathology in day-old chicks. INTRODUCTIONA considerable number of non-typable and autoagglutinable strains of E. coli has been isolated lately from clinically affected birds as well as from poultry dust. Rosenberger et al. (1984) reported that 16 out of 95 E. coli isolates were nontypable and highly or moderately pathogenic to 1-day-old broiler chicks. Nakamura et al. (1982), testing fluctuation of drug-resistant E. coli strains, has isolated some non-typable forms from the fluff of hatchers and from day-old chicks. It is known that E. coli bacteria undergo smooth to rough (S to R) mutation and that this change results in greater or lesser loss of polysaccharide which determines 0 serotype. Rough forms are characterised by their ability to agglutinate spontaneously in the culture medium or serum (Edwards and Ewing, 1968).