A specific rabbit antibody was prepared against chick embryo liver mitochondrial delta-aminolevulinic acid synthase (EC 2.3.1.37) and used to quantitate the amount of enzyme present in liver mitochondria from normal and drug-treated chick embryos. When increases in enzyme activity were produced by the drugs 2-allylisopropylacetamide, or 1,4-dihydro-3,5-dicarbethoxycollidine, quantitative immunotitrations and the Laurell electroimmunoassay showed proportional increases occurred in the amount of enzyme antigen. Conversely, decreases of induced enzyme activity produced by hemin were accompanied by corresponding decreases in enzyme antigen. The relative rate of synthesis of delta-aminolevulinic acid synthase was measured by pulse-labeling of liver proteins with L-[4,5-3H]leucine. delta-Aminolevulinic acid synthase was isolated by quantitative immunoprecipitation followed by electrophoresis of the dissolved immunoprecipitate on a sodium dodecyl sulfate-polyacrylamide gel. In normal and drug-treated chick embryo liver mitochondria, enzyme activity was closely correlated with the relative rate of enzyme synthesis. When a 360-fold increase in enzyme activity was produced in vivo by a combination of 2-allylisopropylacetamide and 1,4-dihydro-3,5-dicarbethoxycollidine in 12 hours, there was a 500-fold increase in relative synthesis, such that delta-aminolevulinic acid synthase constituted over 1% of the total intracellular protein synthesis and over 3% of the total labeled protein in liver mitochondria. Hemin administered after inducing chemicals was able to completely block the induced synthesis of delta-aminolevulinc acid synthase with no significant change in general protein synthesis. Mitochondria from untreated chick embryo livers contain delta-aminolevulinic acid synthase that migrates on sodium dodecyl sulfate gels identically with the induced enzyme.