Myofibrils of chicken heart cells do not contain the electron-dense material of the so-called m-bridges which transverse the sarcomere in the M-region. It has been shown that the M-isoform of creatine kinase (MM-CK) which is mainly responsible for the m-bridge material is not expressed during differentiation of chicken heart cells. No transition from the embryonic BB-CK to the muscle-specific MM-CK takes place in chicken heart, thus no MM-CK is available for m-bridge formation. Here we report on chicken heart cells microinjected with either MM-CK protein or with poly(A+)RNA enriched for M-CK message. In both cases appearance of MM-CK within the M-band of heart myofibrils could be observed by immunofluorescence, indicating translation of the injected message as well as specific binding of the translation product to the M-band of myofibrils. The M-band protein myomesin which is regularly found in heart myofibrils served as specific marker for assembled myofibrils in double immunofluorescence experiments.