When compared with 40 normal controls, 46 patients with essential cryoglobulinemia had higher serum concentrations of a basement membrane antigen, laminin fragment P1 (LP1). Serum LP1 values were more pronounced in patients with visceral involvement, compared with those with skin vasculitis only. Asymptomatic patients or those with arthralgia usually had normal values. Increased disease activity was associated with a significant increase in LP1 (P less than 0.01). For evaluation of disease activity, determination of serum LP1 was a more sensitive measure than any complement factor assay.