The agglutination by concanavalin A of isolated epithelial cells of the rat bladder was examined after in vivo treatment of rats with various bladder carcinogens for one week. The carcinogens tested were N-butyl-N-(4-hydroxybutyl)nitrosamine, dibutylnitrosamine, N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide, 2-acetylaminofluorene, 2-napthylamine, benzidine, N-methyl-N-nitrosourea, and cyclophosphamide, and they were given to male Wistar rats p.o., s.c., intravesically, or i.p. As negative controls, the effects of administration of 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide, dimethylnitrosamine, N-methyl-N'-nitro-N-nitrosoguanidine, and surgical implantation of glass beads in the bladder were also tested. One week after the start of treatment, epithelial cells were isolated from the bladder by sonication, and agglutination of the isolated cells with concanavalin A was assayed. The observed agglutinabilities of isolated cells were found to be closely correlated with the reported bladder carcinogenicities of these chemicals in rats. Thus, concanavalin A agglutination of bladder cells should be a useful rapid in vivo mammalian system for screening bladder carcinogens.