Tropomyosin kinase is partially purified from 14-day-old chicken embryos using DEAE-cellulose, cellulose phosphate and gel filtration chromatography. The purest enzyme preparation consists of two major bands of Mr = 76,000 and 43,000 on SDS-polyacrylamide gel electrophoresis. The molecular weight of the enzyme is 250,000 determined by gel filtration chromatography. It phosphorylates casein and skeletal tropomyosin equally well but histone and phosvitin at a much slower rate. Smooth muscle myosin light chain, tropomyosin from platelet, erythrocyte and smooth muscle are not phosphorylated. The apparent Km for skeletal alpha-tropomyosin and ATP is 50 microM and 200 microM, respectively. Vmax varies between 100-300 nmol/min per mg depending on the purity of the preparation. Mg2+ and dithiothreitol are essential for activity but Ca+, calmodulin and cAMP are not required. The optimum temperature is 37 degrees C and optimum pH is about 7.5. Heparin, a potent inhibitor of casein kinase II, has no inhibitory effect on the enzyme. Similar tropomyosin kinase activity is not detected in skeletal muscle in adult rabbit and chicken. The tropomyosin kinase described here represents a hitherto uncharacterized kinase responsible for phosphorylation of tropomyosin in the chicken embryo.