Neutrophil granulocyte chemotaxis and intraneutrophilic and plasma levels of lysozyme as well as the number of T and B lymphocytes and lymphocyte transformation in vitro on stimulation with mitogens and microbial antigens were studied in four groups of patients with diabetes mellitus (DM). Twelve patients with insulin-dependent diabetes mellitus (IDDM) and ketoacidosis and 4 patients with non-insulin-dependent diabetes mellitus were studied at the time of diagnosis and before and after start of treatment. Ten patients with IDDM of less than 10 years' duration which had been difficult to regulate well and 10 patients with IDDM well regulated for more than 20 years were studied at their regular outpatient visits. Apart from a slight increase in plasma lysozyme in group 1 from the first to the second examination, we found no differences between diabetics and healthy control persons. It is concluded that if patients with DM are more susceptible to infections, it is probably caused by elements of neutrophil or lymphocyte function not examined in this study or by factors unrelated to immunity.