The relationship between tumour size and the uptake of three radiolabelled anti-CEA localising antibodies (A5B7, 1H12 and PK2G) into a human colon tumour xenograft (MaWi) has been examined. For tumour weights greater than 100 mg (109-873 mg) there was a strong positive correlation between absolute uptake and tumour weight with mean uptakes per gram of 9.8 (r = 0.92), 5.0 (r = 0.93) and 5.3 (r = 0.94) for A5B7, 1H12 and PK2G respectively. For tumour weights below 100 mg (17-99 mg) the percentage uptake per gram (specific uptake) increased markedly reaching 80% of the injected dose for A5B7. The above phenomena could be modelled by representing uptake by the surface area of a sphere and tumour weight by its volume. Transformation of this model produced a linear relationship suitable for regression analysis of the experimental data. The slopes of the regression lines for the three antibodies were very close to that predicted by the model suggesting that their uptake into MaWi xenografts is proportional to surface area. The main discrepancy of the actual data was shown by the intercepts which relate to the variation in uptake between different antibodies. This model provides a possible means of correcting for the effect of tumour size when investigating the uptake of antibodies into xenografts.