CAP and ZOIN were tested in vivo for their ability to induce sister-chromatid exchanges (SCE) and chromosome aberrations in mouse bone marrow cells. Single intraperitoneal injections of ZOIN to a maximum of 3000 mg/kg body weight failed to increase the number of SCEs in metaphases recovered 24 h post-treatment, and doses of 1500 mg/kg did not induce measurable increases in chromosome-aberration levels among first-division metaphases at 18 h. Similarly, neither endpoint showed a significant increase following near-lethal doses of 700 mg/kg of CAP. Two lower doses of each chemical were also ineffective. Under the in vivo conditions of our test system, both chemicals were cytogenetically inactive.