A model of chronic serum sickness (CSS) was established in Wistar rats. Rats were given 2 mg of bovine serum albumin (BSA) i.p. five days/week. A group of six male rats were bred which developed albuminuria after three weeks. Rat albumin was measured by radial immuno-diffusion. Albuminuria increased gradually over the first 80 days. On days 87 and 88 rats were given 25 mg/kg methylprednisolone (MP) i.p. Albuminuria increased from 1.1 mg/ml to 1.7 mg/ml. After proteinuria had returned to pre-treatment values, MP was given again with the same result. From day 115 rats were given 25 mg/kg of CYA i.p. for three consecutive days. Albuminuria fell from 0.85 mg/ml to 0.28 mg/ml, and for the first time rats developed microscopic haematuria. After five days proteinuria returned to former levels but then rose to higher levels. On day 146 rats were given CYA and albuminuria fell from 1.7 to 0.45 mg/ml. On day 150 the renal histology was examined by light microscopy and the glomeruli showed no significant abnormalities.