The extent and nature of intergeneric coaggregations among non-flocculating sludge bacteria were studied through examination of the coaggregation abilities of Acinetobacter johnsonii S35 with two other strains of non-flocculating sludge bacteria (Microbacterium esteraromaticum S38 and M. esteraromaticum S51). At first, the effect of electrolyte concentration as well as the addition of EDTA and proteases on coaggregation were studied. Changes in electrolyte concentration had little effect on the aggregation index (A.I.). However, the A.I. was reduced by 35-45% (addition of 2 mM EDTA) and 15-58% (addition of 0.2 mg/ml Actinase E) within 1 h depending upon the bacterial pair involved. Furthermore, the effect of pretreating either A. johnsonii S35 or its partners with Actinase E and periodate, respectively, was studied. The results indicate that these coaggregations involve a protein on the partner's surface that may interact with carbohydrate or a carbohydrate-containing moiety on the surface of A. johnsonii S35.