The recent development of simple, fluorogenic enzyme assays for infantile and late infantile neuronal ceroid lipofuscinosis (INCL and LINCL; CLN1 and CLN2) has greatly facilitated the diagnostic process for these diseases. In leucocytes and fibroblasts from INCL (n = 38) patients we found profound deficiencies of palmitoyl-protein thioesterase I (PPT1), the residual activity was < 5% of mean control activity. In fibroblasts from LINCL patients we found a similar deficiency of tripeptidyl-peptidase I activity (TPP-I), with < 2% activity in 16 patients. The residual TPP-I activity in leucocytes from LINCL patients seemed substantially higher. We also showed the feasibility of reliable prenatal enzyme analysis. In five first-trimester and two second-trimester prenatal analyses for INCL, four affected foetuses were detected (PPT activity 3-6%). Two first trimester pregnancies at risk for LINCL were analysed and a clear TPP-I deficiency was detected in both cases (TPP-I activity 3-4%). The first patient with adult neuronal ceroid lipofuscinosis (ANCL) due to a deficiency of PPT is presented; her present age is 53 years and the onset of the disease was at 38 years with psychiatric symptoms.