Electrophoresis of extracts of turkey spermatozoa for lactate dehydrogenase activity revealed the usual five tissue LDHs (LDH-1 to LDH-5). 2. The presence of LDH-X (the spermatozoan-specific isozyme) was not obvious. 3. Only one band was present on electrophoresis of fowl spermatozoan extracts and it coincided with LDH-1 (heart type). 4. Kinetic investigations, the use of inhibitors and the heat-stability test confirmed that the fowl spermatozoan LDH was probably LDH-1 and not LDH-X.