SummaryThe biological compatibility of an experimental glass ionomer (silicopolyacrylate) cement has been assessed inin vitro(cell culture) andin vivo(monkey teeth) tests. For comparison a conventional silicate cement (Super Syntrex®) was included in thein vivoexperiments.In vitro, the glass ionomer cement was toxic when freshly prepared. The toxicity decreased, however, with increasing setting time. In experiments with prolonged cell‐material contact time with specimens which had set for 24 h, the material appeared to be non‐toxic. Thein vivoexperiments confirmed previous reports concerning unfavourable pulp reactions caused by silicate cement, while the glass ionomer cement caused mainly a mild pulp reaction after 8 days of observation. Unfortunately no longer‐term observations were obtained since the material had been washed out of the cavities within 3