Cyanobacterial toxins, especially microcystins (MCs), are found in eutrophized waters throughout the world. Acute poisonings on animals and humans have been reported following MC exposure. Around 80 MCs variants have been isolated in surface waters worldwide so far. The toxicity of the most frequent MC congener, MC-LR, is well known; however, studies dealing with MC-RR and MC-YR are less abundant. In this present work, the toxic effects of MC-RR and MC-YR at concentrations of 50, 100, 150 and 200 microM have been investigated in the human colon carcinoma cell line Caco-2 both undifferentiated and differentiated after 24 and 48 h exposure. Toxicity endpoints assessed were cell number by quantification of total protein content of the cell cultures; cell viability by means of neutral red uptake, and 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium inner salt (MTS) metabolization to detect mitochondrial changes. Moreover, morphological alterations were also investigated. Results showed that protein content was the most sensitive endpoint for MC-RR with reductions of 45% after 48 h exposure to 200 microM MC-RR in differentiated cells (EC(50)>200 microM); whereas for MC-YR is the inhibition of neutral red uptake with reductions higher than 80% at 100 microM in undifferentiated cells after 48 h (EC(50) of 57.3 microM). Furthermore, alteration in the cells was shown in the morphological studies, particularly at high concentrations, undergoing general reduction in cell number and hydropic degeneration. The sensitivity of the cultures to these toxins was highly affected by the exposure time and in a lesser extent by the differentiation state, with MC-YR showing higher toxicity than MC-RR.