Eight strains of Actinomyces were examined for their ability to coaggregate in vitro with four strains of Candida albicans. The Actinomyces coaggregated to various degrees with all of the Candida strains. Exposure of the Candida but not the Actinomyces to heat, trypsin, proteinase K, amphotericin B or trichodermin abolished coaggregation. All sugars tested did not inhibit any of the reactions. All coaggregating pairs were disaggregated by the addition of SDS, but nonionic detergents had no effect. The addition of urea or EDTA completely reversed coaggregation. Actinomyces strains were sensitive to periodate oxidation, whereas the Candida strains were unaffected. These data suggest that the coaggregations involve a protein on the Candida surface that may interact with carbohydrates or carbohydrate-containing molecules on the surface of the Actinomyces. These observations expand the known range of intergeneric coaggregations occurring between human oral microbes and indicate that coaggregation of C. albicans and Actinomyces may be an important factor in oral colonization by this yeast.