Cold-reacting serum lymphocytotoxic antibodies (LCAs) were measured in sera from 230 insulin-dependent juvenile-onset diabetes mellitus (IDDM) patients and from 116 control subjects. LCAs were present in only 4% of control sera compared with 19% in IDDM patients. The most significant determinant of LCAs was time since onset of diabetes; within the first 12 mo, 55% of IDDM sera had LCAs, compared with 25% after one year and 15% after five years of diabetes. LCAs were absent in sera from patients with IDDM for 10 yr or more. Genetic factors were also implicated in susceptibility toi occurrence of LCAs. HLA antigen B8 and B18 were associated with an increased risk for LCAs, whereas HLA-B7 was associated with a decreased risk. The relative risk for LCAs in patients positive for HLA-B8 but not B7 was 2.3, compared with 0.0 in HLA-B7/B8 heterozygotes. In contrast, B7 did not provide protection from LCAs in B18/B7 IDDM patients. Properdin factor B (Bf) alleles, which are in linkage disequilibrium with alleles of the HLA-B locus, were also associated with LCAs, IDDM patients with alleles BfS1 or BfF hd a prevalence of LCAs of 7%, significantly less than the 39% in Bf-F1S or -F1 patients. LCAs were not identical or closely correlated to pancreatic islet cell antibodies. Our findings indicate genetic heterogeneity in, yet, another autoimmune process in IDDM.