The cyanobacterial toxin cylindrospermospin (CYN) was shown to inhibit the eukaryotic protein synthesis apparatus with similar potency in plant and mammalian cell extracts, IC50 of 334 nM in wheat germ extract and 110 nM in reticulocyte lysate. [14C] CYN binding was assessed in reticulocyte lysates, following separation of ribosomes from free toxin on Sephadex G-50 size exclusion columns. While toxin binding was shown to be concentration-dependent (60-600 nM [14C] CYN), it did not correlate with ribosome content. The molar ratio of toxin to ribosomes was 0.02:1 at the binding plateau. Significant binding of [14C] CYN was identified in both the ribosome fraction (>90% total ribosomes) and the first wash fraction eluting from the Sephadex G-50 column, despite the low ribosome content (<10% total ribosomes) in the latter. Bound [14C] CYN could be partially displaced by incubation with an excess of unlabelled CYN in both the ribosome and wash fractions, indicating a non-covalent linkage. Molecular weight cut-off filters identified that [14C] CYN was associated with high molecular weight material >100 kDa. These results suggest that CYN's target may not be the ribosome itself, but rather one of the soluble proteins associated with the eukaryotic translation system.