For a detailed analysis of the mechanism of extracellular CaZt effects of MM-CK* accumulation during myogenesis in uitro (Morris & Cole, 1979), immunological techniques are almost essential. A radioimmunoassay specific for the M-subunit of CK would obviate the necessity of separating isoenzymes on gels to distinguish embryonic and muscle-specific forms (Morris et al., 1976). Potentially, radioimmunoassay can also detect inactive forms of the enzyme, allowing inactivation processes to be studied. The specificity of the reaction would allow measurement of chick CK in the presence of CK from another species, e.g. CK released into complete culture medium containing horse serum. Such studies of enzyme release and inactivation may have a bearing on the release of MM-CK from muscle in certain muscle diseases and on the use of serum CK as a diagnostic test (Pennington, 1980). Chick CK was prepared using the method of Eppenberger et al. (1967). Purity was demonstrated by obtaining a single band when the preparation (1OOpg) was run either on gels without sodium dodecyl sulphate (Davis, 1964) to separate the isoenzymes (Morris & Cole, 1979) or on the same gels with 0.1% sodium dodecyl sulphate and 10mM-/-mercaptoethanol added. Antisera were raised in rabbits by intradermal injection of enzyme in Freund's adjuvant. The antiserum gave a single precipitin band on Ouchterlony gels with both pure enzyme and whole muscle cell extract. Immunofluorescence microscopy of 4-day muscle-cell cultures revealed almost exclusive reaction of the antiserum with myotube cytoplasm, consistent with