Sublethal toxicity studies were conducted with a rotifera (Brachionus calyciflorus) to determine the No observed effect concentrations (NOECs) of a different eight toxic substances based on the protein mass reduction. Protein mass (with moleculer weight 43KDa - 67KDa) was quantified using electrophoresis method. Quantitation of protein concentrations demonstrated that toxicant stress reduced protein concentrations. The NOECs-43KDa and NOECs - 67KDa were reported. Since in vivo protein cocentrations can be assessed in a short time, these results can be obtained in only a few hours, which is substantially less than traditional whole animal reproductive tests. NOECs - NOECs - 43KDa test were generally more sensitive end points than NOECs - NOECs-67KDa test and more sensitive than standard whole animal toxicity tests for only four toxicants.