Metastudies have found no consistent effects of the cyanobacterial toxin microcystin on Daphnia, and there are discrepancies between field observations and experiments. Confounding factors include absence or presence of alternative high quality food or the presence of bioactive compounds, other than microcystins in cyanobacteria. Of specific interest are lipopolysaccharides (LPS) on the outer cell wall. LPS may have a number of biological effects, including reduced detoxication of microcystins in plants and animals. When grazing seston in the field, filterfeeders take up heterotrophic bacteria attached to cyanobacteria, as well as free-living bacteria. The LPS produced by heterotrophic bacteria have been shown to be much more harmful than cyanobacterial LPS. We performed two experiments in which we tested for potential synergistic effects between bacterial LPS and microcystins. Full-factorial experiments separated the main effects and interactions between (i) food quantity as well as food quality (addition of the green alga Scenedesmus),(ii) presence or absence of strains that vary in amount and composition of microcystins (microcystin free strain NIVA-CYA43, moderate microcystin producing strain NIVA-CYA140 and high microcystin producing strain PCC7820), and (iii) presence or absence of bacterial LPS on different life history traits of Daphnia galeata. We measured juvenile growth rate, age and size at first reproduction, death before first reproduction and standard carbon content of Daphnia. From the experiments we conclude that microcystin-producing Microcystis had deleterious effects on the life history of D. galeata, but especially when the availability of high quality green algal food was limited in comparison to the supply of microcystin producing strain PCC7820. In the experiment in which PCC7820 was used as microcystin-producing strain, addition of LPS lowered SCC of Daphnia, but had no effects on other life history parameters. The interaction between Microcystis strain, Microcystis concentration and LPS was highly significant in case of PCC7820, but not in case of CYA-140, indicating that the effects of LPS and its interactions with microcystin on Daphnia life history were strongly context dependent. (C) 2011 Elsevier B.V. All rights reserved.