Extracts, possibly containing a factor(s) which agglutinates spores of Ceratocystis fimbriata, were obtained from the sweet potato, potato, taro, cucumber and kidney bean. The factor(s) seemed to be involved in the expression of host specificity by various strains of C. fimbriata. The agglutinating factor(s) from sweet potato root had a high molecular weight and was inactivated by pronase or macerozyme treatment.