Cylindrospermopsin (CYN) is a cytotoxin produced by different cyanobacterial species, increasingly detected in water reservoirs worldwide. There is very little information available concerning the effects of the toxin on fish immune cells. The aim of the study was to elucidate the potential impact of cylindrospermopsin on the selected parameters of a common carp (Cyprinus carpio L.) leucocyte cell line (CLC). The cells were incubated with the cyanotoxin at concentrations of 10, 1 or 0.1 µg ml–1 for up to 48 h. Cell viability and proliferation, apoptosis/necrosis induction, cell morphology and phagocytic activity were determined. The two higher toxin concentrations occurred to be evidently cytotoxic in a time-dependent manner and influenced all studied parameters. The lowest used concentration had no effects on cell viability and cell number; however, a strong reduction of bacteria uptake after 24-h exposure was detected. The obtained results indicate that cylindrospermopsin may interfere with the basic functions of fish phagocytic cells and as a consequence influence the fish immunity. Copyright © 2014 John Wiley & Sons, Ltd.