Spontaneously occurring coaggregation-defective (COG-) mutants of oral actinomycetes and streptococci were isolated and used to study interactions between cells of these two kinds of bacteria. COG- mutants of each kind of bacteria were isolated by a simple enrichment scheme. Parent strains were mixed with a coaggregating partner strain, coaggregated cells were removed by low-speed centrifugation, and non-coaggregated cells were recycled by the addition of more partner strain cells. COG- mutants constituted up to 10% of the parent strain cell type in the final enriched cell suspension. Unlike their respective parent strains, COG- mutants of Actinomyces viscosus T14V and Actinomyces naeslundii ATCC 12104, and A. naeslundii I exhibited no lactose-reversible coaggregation with certain oral Streptococcus sanguis strains. However, these COG- mutants were not altered in their coaggregations with another S. sanguis strain, H1, a member of a streptococcal coaggregation group that exhibits only lactose-nonreversible coaggregations with oral actinomycetes. Although all coaggregations between S. sanguis H1 and these actinomycetes appear to be alike, examination of a COG- mutant of S. sanguis H1 revealed that, like its parent, it coaggregated with A. viscosus T14V and its COG- mutants, but unlike its parent, it did not coaggregate with the two A. naeslundii strains or their COG- mutants. Thus, it was concluded that at least two types of surface components are important in mediating coaggregation between S. sanguis H1 and actinomycetes. The COG- mutant of S. sanguis allowed detection of these components, which were indistinguishable in previous studies.