Residual adenylosuccinase activity was studied in cultured lymphoblasts from a pair of siblings with infantile autism who have been previously shown to have a deficiency of the enzyme. The rates and distribution of de novo purine synthesis by intact cells were nearly normal. There was no evidence of inhibitory activity in the lysates of the mutant cells. The optimal pH was indistinguishable from that in control cells. The apparent Km in the two mutant cells lines is not significantly different from normal, but the mutants displayed markedly decreased maximum steady-state velocities. Residual activities in mutant cells show decreased thermal stability, suggesting that there is a structural mutation of the adenylosuccinase in the mutant cells.