The expression of the intermediate filament (IF) constituents desmin, vimentin and keratin, as well as the striated-muscle-specific marker titin, was studied in mouse embryos of 8.0 to 9.5 days post coitum (d.p.c.), using the indirect immunofluorescence technique in combination with polyclonal and monoclonal antibodies. During the development of the embryo, desmin was first detected at 8.25 d.p.c. in the ectoderm, where it was transiently coexpressed with keratin and vimentin. At later stages, the ectoderm contained only keratin and to a certain extent also vimentin IF. At 8.5 d.p.c., desmin was found exclusively in the heart rudiment, and remained present with increasing intensity in the myocardial cells during later cardiogenesis. Striation of desmin in the heart muscle cells was observed in 9.5 d.p.c. embryos. At these stages (8.5-9.5 d.p.c.), triple expression of the IF proteins desmin, vimentin and keratin was evident in these cells. From 9.0 d.p.c. onwards, desmin could be detected in the myotomes as well. Immunoblotting studies of 9.5 d.p.c. mouse embryos confirmed the immunohistochemical data. Titin was found in the early heart anlage at stage 8.25 d.p.c., when no desmin expression was observed in this tissue. At this stage the titin appeared in a punctate pattern, similar to that observed in cardiac myofibrils of early chicken embryos (Tokuyasu and Maher, 1987; J. Cell Biol. 105, 2781-2793). In 8.5 d.p.c. mouse embryos, this punctate titin staining pattern was still observed, while, at this stage, a filamentous staining reaction could be seen with the desmin antibodies. During further development, cross-striation was detected within myocardial cells using the polyclonal titin antibody from 9.0 d.p.c. onwards, i.e. before such striation could be detected with the desmin antibodies. From these data, we conclude that titin synthesis may anticipate desmin expression in the developing mouse myocard, although the level of expression of the former protein remains low until 9.0 d.p.c.