Malathion administration at non-cholinergical doses was shown to elevate macrophage, proliferative and humoral immune responses. This study examined the effects of malathion on autoimmunity, autoantibody formation, macrophage function and mitogenic responses in MRL-lpr mice (genetically predisposed to autoimmune disease) and MRL-+/+ mice. Malathion, 33-300mg/kg, was administered by gavage once per week, beginning at 6 weeks of age. At 300mg/kg in MRL-lpr mice, malathion administration accelerated the appearance of significant (>100mg/l) levels of urinary protein by approximately 3 weeks and increased the maximum level of protein detected. Increased urinary protein was delayed at lower doses of malathion, but was elevated compared to vehicle control. This increase in urinary protein was not observed in the group of MRL-+/+ mice. The popliteal and axillary lymph nodes (LN) were larger in malathion-treated (>33mg/kg) than in control mice at 19 weeks of age. Within the same time-frame in MRL-+/+ mice, malathion did not affect and increased the size of the axillary and popliteal LN, respectively. Rheumatoid factor (RF) and anti-DNA (dsDNA) antibodies in the serum were not elevated in any group of MRL-+/+ mice by 19 weeks of age. However, in the MRL-lpr mice, weekly malathion treatment (>33mg/kg) elevated the level of serum RF at 12 and 19 weeks of age. Malathion treatment (>100mg/kg) also increased the level of anti-dsDNA antibodies in the serum of MRL-lpr mice at 19 weeks of age. Malathion treatment increased the number of inflamed glomeruli. Histopathological analysis of various organs showed no effect on vasculitis after malathion treatment. Acute administration of 300mg/kg malathion to 6-week-old mice elevated the secretion of nitric oxide by peritoneal macrophages, but did not affect the secretion of tumor necrosis factor. In addition, the basal and mitogen-induced proliferation of splenocytes of malathion-treated MRL-lpr mice were elevated, but the stimulation index was unchanged.