Radio-labelled antimyosin-monoclonal antibodies (AMA) have been introduced to demonstrate myocardial necrosis after cardiac infarction or in cardiac allograft transplants. As rhabdomyosarcoma (RMS) and leiomyosarcoma (LMS) are tumors of myogenic origin, thus often containing myosin, we decided to use the 111In-labelled Fab fragment of AMA (Centocor) in scintigraphic tumor detection. We examined 13 children with histologically-confirmed RMS and LMS, and five other children with other types of soft tissue sarcomas. Conventional techniques were used to determine the extent of the tumor. An uptake of the tracer was observed in all known tumor sites in seven RMS patients. As the scans were negative in three RMS patients who were in complete remission (CR) and in two other patients (fibrosarcoma and haemangiopericytoma) with a measurable tumor mass, we considered them to be "true negative". In the three remaining CR patients (1 RMS, 2 LMS) the scans were positive but weak in the primary tumor site in two cases and in a distant site (bone) in the third respectively. We considered them to be "false positive" as no tumor cells were evident in the biopsy specimen. In one case, the antimyosin uptake was presumably the result of damage to the muscles after radiation. Interestingly, in three patients with other malignancies such as rhabdoid and peripheral neuroectodermal tumors there was a noticeably strong uptake of the tracer in the primary tumors and the scans turned negative after complete remission was achieved. The diagnostic AMA scanning showed no side-effects. The reason why antimyosin antibodies permeate the membrane of the tumor cells is yet undetermined.(ABSTRACT TRUNCATED AT 250 WORDS)