The high molecular weight (M(,r) (TURN) 1 x 10('6)) protein titin was isolated from purified bovine longissimus muscle myofibrils by gel filtration column chromatography in the presence of sodium dodecyl sulfate (SDS). Purified bovine, rabbit and chicken skeletal muscle titins were closely related or homologous in that they yielded similar column elution profiles and had similar amino acid compositions. Titin purified from rabbit and bovine skeletal muscles also comigrated in SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and cross-reacted with antibodies specific for titin. Indirect immunofluorescence localization demonstrated that titin is preferentially localized in the A-I junctional area with some diffuse staining throughout the entire A-band in bovine and porcine longissimus muscle myofibrils. As analyzed by SDS-PAGE, the quantity and electrophoretic mobilities of titin and nebulin from bovine longissimus dorsi, biceps femoris and psoas major whole muscle samples and purified myofibrils were similar, indicating there were no variations in these two proteins with different muscles. There were also no differences in the quantities or electrophoretic mobilities of titin and nebulin from red (chicken mixed thigh) muscle and white (chicken pectoralis major) muscle. Although there were no species variations in overall column elution profiles, the amount of titin and nebulin degradation occurring in myofibrils from at-death bovine longissimus, porcine longissimus, chicken mixed thigh and chicken pectoralis major muscles varied with both species and muscle fiber type;Purified bovine longissimus muscle myofibrils and whole muscle samples were prepared from at-death muscle and from muscle stored at 2, 25 and 37(DEGREES)C for 1, 3, and 7 days postmortem, and were analyzed by SDS-PAGE. Titin migrated as a closely spaced doublet in samples from at-death muscle. With increased postmortem storage time and temperature, the top band of the titin doublet gradually disappeared. Only the lower doublet band (putative breakdown product of upper band) remained after 7 days storage at 2 or 25(DEGREES)C. A trace of the lower band was present by 3 days of storage at 37(DEGREES)C along with a new putative degradation product immediately beneath this band. Thus, titin was degraded in postmortem muscle, and the rate of degradation was enhanced by increases in storage time and temperature.