Ten adult, female cynomolgus macaques were randomly assigned to two equal groups: (1) semipurified diet (SPD); and (2) SPD with 45% ground alfalfa seed (AS). Both groups were studied at monthly intervals after 5 mo on their respective diets. Control animals had a mean hematocrit (Hct) of 43 +/- 2%, negative antiglobulin (AG), antinuclear antibody (ANA) and LE cell tests. Mean values for C3 and C4 were 309 +/- 47 mg/dl and 35 +/- 7 mg/dl, respectively. Mean serum binding to radiolabeled double stranded deoxyribonucleic acid (dsDNA) was 1.9 +/- 0.2%. Three of five animals fed AS developed signs of an SLE-like illness characterized by AG-positive anemia (lowest Hct 30%), positive ANA (highest titer greater than 1:15, 360; rim pattern) and elevated anti-dsDNA binding (highest 96%) with variable degrees of hypocomplementemia. One animal had granular deposition of immunoglobulin and complement at the dermal-epidermal junction of clinically normal skin the presence of immune complex-induced glomerulonephritis.