The impact of microcystin-LR, one of the most common cyanobacterial toxins, on liver and gut cells originating from mammals and fish was compared. Upon exposure of human and rainbow trout (Oncorhynchus mykiss) cell lines up to 2.5 microM microcystin-LR, no alteration in cell viability was observed as assessed with three fluorescent indicators dyes, CFDA-AM, Alamar Blue and neutral red. The lack of sensitivity of the trout cell lines coincided with an absence of detectable mRNA levels of organic anion transporter polypeptide (OATP), which is implicated in the uptake of microcystin-LR. In contrast to the cell lines, primary rainbow trout and mouse hepatocytes showed damage to subcellular structures, particularly the lysosomes, as indicated by neutral red. This led us to propose a thus far undetected role of lysosomes as targets and mediators of microcystin-LR elicited cellular damage. An inhibitor of OATP, rifampicin, partly protected hepatocytes from this damage. Yet, the sensitivity of rainbow trout hepatocytes rapidly declined in culture, accompanied by decreasing levels of OATP mRNA. The sensitivity of mouse hepatocytes toward microcystin-LR also declined in culture but overall was about 25-fold greater than that of the trout cells. These differences mirror those observed in vivo and suggest the use hepatocytes for deciphering the species differences.