In a single blind randomised cross-over study, 40 patients were changed from ordinary bovine to highly purified porcine insulins for a period of 6 months. Half were later rechallenged with bovine insulin. Sequential determinations of IgG insulin binding capacity for bovine insulin were correlated with insulin dose and diabetic control. After changing to highly purified insulins the following correlations were observed between percentage change in insulin dose and change in insulin binding capacity: at 2 months r = 0.35 (p less than 0.05), at 4 months r = 0.38 (p less than 0.02) and at 6 months r = 0.37 (p less than 0.02). When the patients who showed substantial changes in HbA1 were removed from the analysis, the remaining 29 demonstrated a clearer relationship between these two variables (r = 0.56, p less than 0.01). Removal of patients with a low initial insulin binding capacity left 18 patients with stable diabetes, and changes in insulin binding capacity and insulin dose showed an even closer correlation for this group (r = 0.77, p less than 0.001). A similar degree of positive correlation was observed after rechallenge with bovine insulin. We conclude that the level of circulating insulin antibody affects the dose of insulin required to maintain stable diabetic control.