Summary Tumour localisation of anti-tumour antibodies varies greatly between patients. Factors which may be responsible for this have been investigated in 56 patients with colorectal carcinoma with a view to improving radioimmunotherapy. Thirty-seven to seventy-four MBq of 1254-labelled mouse monoclonal antibody to CEA, was given intravenously and tumour resected 70-480 h later. Percentage injected activity kg-' (% inj.act kg-') in tumour, was inversely correlated with the time interval between injection and operation (P = 0.004). To assess the influence of other parameters on localisation, patients were divided into two time groups according to time interval between injection and operation, 70-120 h (n = 33) and 144-480 h (n = 23). In neither group was there a significant correlation of % inj.act kg-' with time. The % inj.act kg-' in tumour showed a significant correlation with that in the blood for both groups (P = 0.005 and P = 0.01). There was no significant correlation for either time group between % inj.act kg- ' (Begent et al., 1985;Martin et al., 1988;Begent, 1990). There is great variability in uptake of antibody in individual tumours in patients and the factors influencing this are poorly understood. Distribution of injected antibodies within tumours is not uniform (Griffith et al., 1988;Pedley et al., 1990