In crop carbon budget studies losses of root material during storage and washing of samples may cause considerable errors. To correct data from field experiments where rhizosphere C fluxes in wheat and barley were determined by C-14 pulse-labelling at different development stages, experiments were performed to quantify losses of C-14 from roots during washing. Losses of C-14 from wheat roots grown on nutrient solution and stored in different ways, decreased from on average 45% of total C-14 content 8 days after labelling to 27% after 21 days. This decrease was probably related to the incorporation of C-14 into structural compounds. During washing of oven-dried soil cores of field-grown wheat and barley 3 weeks after labelling, different size classes of losses of C-14 from the roots increased substantially with the development stage of the crop at labelling. The 0.3-0.6 mm size class increased from 5% of the C-14 in roots > 0.3 mm in young plants to 25% at ripening, and the < 0.3 mm size class increased from 8 to 41% of total C-14 content. The latter size class was, however, determined by washing handpicked roots and may therefore partly consist of adhering exudates, mucilages and microorganisms. The effect of development stage on root washing losses was attributed to root senescence which increases the fragility of roots. Thus, especially at the late development stages root washing losses caused a severe underestimation of the root C-14 content. However, with these results the C-14 distribution patterns of the field experiments could be adequately corrected.