Intercellular communication (IC) was investigated by a dye transfer method in 5 human bladder carcinoma cell lines, JTC-29, JTC-30, JTC-32, HUB-41 and T-24 which showed various histological anaplasia when transplanted into nude mice. IC was investigated as follows; 1) IC among cells of each bladder carcinoma cell lines, 2) IC between various combination of different cell lines and 3) IC between cells of bladder carcinoma cell lines and fibroblasts prepared from the human skin or prostate. High IC was observed among JTC-30 cells which showed a differentiated pattern when transplanted into nude mice. JTC-32 and HUB-41, of which transplanted tumors showed poorly differentiated patterns, demonstrated lower IC than JTC-30. Statistical significance was present only between JTC-30 and JTC-32 cells (p less than 0.05). T-24 which formed histologically anaplastic tumor in nude mice demonstrated markedly poor IC. Second, IC between two different cell lines was demonstrated only between JTC-30 and JTC-32. Third, fibroblast from the skin and prostate had high IC but showed apparent IC with none of bladder carcinoma cell lines. These findings indicate that ICs of bladder tumor cells vary with cell lines tested and appear to decrease with progress of malignant anaplasia. Furthermore, the fact that there is sometimes high IC and sometimes lack of IC between differentiated and undifferentiated tumor cells suggests a possible role of IC in prevention of malignant progression. The present study provides no evidence of IC between bladder tumor cells and fibroblasts, which in some reports have been suggested to be an important phenomenon in evaluating the metastatic capacity of cancer cells.