5-thio-D-glucose (5-thio), a closest chemical analog to normal D-glucose has been found to be an effective reversible male contraceptive in mice and gerbils. In this study, several screening methods will be utilized to test the possible mutagenic potential of this contraceptive in male mice. Adult, male albino STS mice will be fed 0 (control), 40 or 60 mg 5-thio/kg daily, and groups of animals from each dosage level will be killed and breeding cages will be set up weekly during the experimental period of 5 weeks for cytogenetical and dominant lethal mutation test (DL) studies, respectively. After this period, food containing 5-thio will be replaced by standard chow and the mice will be tested biweekly for 10 weeks for cytogenetical and DL effects. Cytogenetic observation will be made to study the effects of this chemical on the numbers, structure and behavior of chromosomes in bone marrow and primary spermatocytes. To determine the antimitotic activity and the chemical's effect on actively dividing cells, mitotic index from bone marrow will be calculated. The influence of this agent on germ cells will be measured by the cell survival of types A and B spermatogonia and resting spermatocytes. Dominant lethality is believed to be the result of gene mutation and chromosomal abnormalities in parent germ cells. In DL test, intra-uterine deaths of fetuses in normal females mated with 5-thio fed males will be scored to calculate the mutagenic index. The experimental results will be compared with those of the control values and the data will be analyzed using appropriate statistical methods.