This study was undertaken to investigate the molecular basis of the two different clinical phenotypes (acute and chronic forms) of type 1 tyrosinemia (fumarylacetoacetase deficiency). Fumarylaceto-acetase (FAA) was isolated from beef liver and antibodies raised in rabbits. Analysis of fibroblasts extracts by immunoblotting showed tne absence of cross-reacting material in cells from acute patients and reduced amounts in cells from chronic patients. Fibroblasts from controls and from both acute and chronic patients were pulse-labeled with 35S-methionine followed by a chase of 1-4 days. Radioactively labeled FAA was immunoprecipitated with proteinA-coupled antibody, dissociated and subjected to SDS-PAGE followed by fluorography. In control fibroblasts after pulse-labeling two bands could be visualized, the upper band having a molecular size of 41.200 daltons, the lower band 0.5-1.0 kilodaltons smaller. These bands disappeared after 4 days. In fibroblasts from acute patients the M=41.200 band after synthesis disappeared within 1 day while in cells from chronic patients the rate of disappearance was in between. These results indicate that the acute and chronic forms of type 1tyrosinemia are caused by different types of mutations.