Serum sickness followed the administration of anti-lymphocyte globulin to a patient with multiple sclerosis. In addition to other characteristic features of this syndrome, there was hypocomplementemia and transient renal dysfunction similar to that observed in the ‘one-shot’ experimental model of serum sickness. Cryoglobulinemia was transiently demonstrable at the height of the inflammatory response. Analysis of the purified cryoprecipitate revealed the presence of human IgG and IgA, and, in addition, equine IgG. This demonstration of a well-defined exogenous antigen in the cryoprecipitate provides further support for the immune complex nature of mixed cryoglobulins.