Blastocystis hominis isolates from asymptomatic carriers and symptomatic patients were cultured in vitro, purified from the co-cultivated bacterial flora and tested for cytopathic effects on monolayers of Chinese Hamster Ovary (CHO) cells and Adeno Carcinoma HT29 cells. In the case of the CHO cells, living B. hominis cells and B. hominis cell lysates were able to cause significant cytopathic effects, which were dependent on the concentration of cells employed. Destruction of the cell monolayers was observed to the same extent with patient isolates derived from healthy or symptomatic B. hominis carriers. HT29 cells were less susceptible: B. hominis cells and cell lysates caused only minor effects which were not statistically significant. Culture filtrates of B. hominis exhibited cytopathic potential on CHO and HT29 cells; however, the control which consisted of filtrates from Robinson's cultures in which B. hominis failed to grow showed similar effects, too. Therefore the culture supernatants could not be proven to produce a specific cytopathic effect on CHO and HT29 cells.