Mustard (Sinapis alba L.) seeds were cultivated for seven days on a solid nutrient medium supplemented with 0-40μg microcystin-RR per ml. Microcystin-RR affected seedling growth (ic50 0.8μg/ml) and microcystin concentrations ≥5.0μg/ml produced malformed plants. The inhibition of protein phosphatase 1 and 2A activity correlated with the growth inhibition. The seedlings were also shown to take up 3H-dihydromicrocystin-LR derived radioactivity up to a level corresponding to ca. 80ng toxin per mg plant protein. Copyright (C) 1998 Elsevier Science Ltd.