Epidemiological studies have reported an increased risk of leukaemia in children who are exposed to extremely low-frequency (ELF) magnetic fields (MF), suggesting that ELF MFs may be carcinogenic to humans. No carcinogenic effects have been found in animal studies that have tested ELF MFs alone. Similarly, genotoxicity studies have generally not shown effects from MFs alone. However, ELF MFs have been reported to enhance the effects of known carcinogenic or mutagenic agents in a few animal studies and in several in vitro studies. This paper discusses the findings of studies on such combined effects. The majority of in vitro studies have reported positive findings, which supports the conclusion that MFs of 100 microT or higher interact with other chemical and physical agents. Further studies should address biophysical mechanisms and dose-response relationship below 100 microT. Animal studies designed according to the classical initiation-promotion concept may not be sufficient for studying the cocarcinogenic effects of MFs, and further studies using novel study designs would be useful. Epidemiological data on the interaction between MFs and other environmental agents are scant and inconclusive, and any further studies may be difficult because of the scarcity of subjects with suitable combined exposures.