1. The present work aims to find a biochemical criterion for evaluating the evolution of sperm according to age through the study of the ATPase activity from the spermatozoa and the acid phosphatase from the seminal plasma of cocks from three different breeds. 2. The optimal parameters of action of the cock semen acid phosphatase and the Ca(2+)-dependent ATPase from the spermatozoa were studied. 3. The substrate specificity of the semen acid phosphatase and its inhibition by tartrate, fluoride, metavanadate, molybdate and Hg2+ were also studied. 4. The two enzymes were determined from the Sussex, Golden Cornish and Plymouth Rock breeds at different ages. 5. The data lead to the conclusion that some properties of bird spermatozoa are less influenced by breed while the acid phosphatase activity, secreted in the ductus deferens is a breed characteristic.