Rabbit myosin prepared in the conventional manner by repeated precipitation at low ionic strength was recently shown to contain substantial amounts of impurities; the principal impurity is a component of the myofibril called C-protein. Because antiserum to such conventionally prepared myosin has been used in the past for labelling studies of muscle, it was necessary to study the immunological characteristics of myosin and C-protein and in particular to test the specificity of this antiserum. Antisera to both rabbit myosin and C-protein have been successfully elicited in goats. These antisera have been analysed by immunodiffusion and by precipitin reactions in solution. The analysis has been helped by the examination of immunoprecipitates by polyacrylamide gel electro-phoresis in the presence of sodium dodecylsulphate. It is concluded that: (a) C-protein and myosin are antigenically distinct and therefore that C-protein is not derived from myosin. (b) Purified myosin can behave as a classically simple antigen giving a single precipitin line when diffused against its homologous antiserum. (c) C-protein is a powerful immunogen; the amount present as an impurity in myosin prepared in the conventional way by repeated precipitation at low ionic strength is capable of eliciting a large amount of antibody. Consequently the pattern obtained by labelling myofibrils with antiserum to conventionally prepared myosin would contain information about the location of C-protein superimposed on information about the location of myosin.