Anti-plasma protein C monoclonal antibodies were prepared and characterized, and quantitative immunoblotting techniques were developed to determine the molecular forms of protein C in whole plasma. Two antibodies reacted with the heavy chain of protein C, four reacted with the light chain, and two reacted only with nonreduced protein C. A doublet of protein C (MW = 63-66K) was seen on nonreduced immunoblots of normal plasma and 30 heterozygous protein C deficient plasmas (2-77% protein C antigen). In reduced plasma, approximately 75% of protein C presented as doublet heavy chains (MW = 39-42K) and doublet light chains (MW = 22-25K), and approximately 25% was single chain (MW = 64K). The immunoblotting technique was quantitative, specific, sensitive, and correlated with electroimmunoassay results. It also provided visual qualitative information not obtainable with other methods of quantitation.