The effect of seminal fluid upon human spermatozoa was analyzed using in vitro chromatin decondensation and automated image analysis. A number of specimen portions processed after incubation in seminal fluid showed different total mean areas as compared to the corresponding portions processed immediately. Comparison of the results obtained with and without delay showed that the incubation in seminal fluid promoted decondensation in some cases, but retarded it in others. Thus, the seminal fluid stabilized the chromatin condensation in some spermatozoa, but not all. The stabilization may be due to the influence of prostatic zinc.