Determining pollutant concentrations in the tissues of experimental test organisms is necessary for understanding uptake and excretion mechanisms of toxicants. Using small organisms can make the determination of organism biomass inaccurate. We here propose the use of selected tissue element contents as a proxy for tissue biomass. Forty different elements were determined in tissues of the two worm species Enchytraeus crypticus and Caenorhabditis elegans derived from cultures exposed to combinations of varying temperatures and sublethal concentrations of Cu and Cd. Three criteria were used to select good biomass indicators: The element concentration must (1) be present in concentrations above the limit of quantification of the analytical method, (2) must be stable and (3) must not be affected by the treatment. If the organisms are believed to have significant amounts of soil in their gut, the element must also be present at higher concentrations in the tissue compared to the soil. The three elements K, Mg and P all lived up to the first three criteria for both worm species, showing correlation coefficients between element content and tissue biomass of 0.97, 0.96 and 0.97 (n = 25) and 0.997, 0.998 and 0.992 (n = 10) for K, Mg and P in the E. crypticus and C. elegans, respectively. Only P would be an appropriate biomass indicator for organisms with a soil gut uptake assuming the tissue concentrations in soil eating organisms are similar to those measured in the present study. Using Mg as a biomass indicator on a verification dataset of Cu and Cd uptake in E. crypticus, compared to giving Cu and Cd content per individual organism, decreased the coefficient of variation from 31 +/- A 21 to 21 +/- A 17 % and from 34 +/- A 22 to 9.3 +/- A 6.4 % for tissue Cu and Cd, respectively. We therefore conclude that the use of an element as a biomass indicator can reduce tissue concentration variability.