Avermectins are a complex of chemically related naturally occurring macrocyclic lactones which first isolated as a natural fermentation product of a soil actinomycete, Streptomyces avermitilis, in 1976 (Burg et al. 1979). The naturally occurring avermectins can be separated into four homologous pairs, of which one, abamectin (avermectin B1), which consists of 80% of avermectin B1a and 20% of avermectin B1b has been developed for use against arthropod pests of horticulture and agronomic crops (Dybas and Green 1984). Currently, it is rapidly worldwide growing in usage for many agronomic crops. They are potent chemotherapeutic agents against animal parasitic nematodes and many veterinary and agricultural arthropod pests (Putter et al. 1981, Campbell et al. 1983, Lasota and Dybas 1991). Abamectin increases calcium permeability to chloride ions, which is independent of the neurotransmitter, gamma aminobutyric acid (GABA)-mediated chloride ion channels (Calcott and Fatig 1984). The compounds' mechanism of toxicity in mammals is unknown but GABA is a mammalian central nervous system neurotransmitter and effects on GABA may be relevant to their safety in mammals (Lankas and Gordon 1989). Carcinogenicity studies with abamectin at maximum tolerated dosage levels for 22 and 24 months duration in mice and rats, respectively, have demonstrated the lack of carcinogenicity of this compound (Lanks and Gordon 1989). Also they showed the lack of genotoxic activity of abamectin in a variety of assays. Abamectin degradates were tested and found to have no adverse maternal and developmental effects in CF 1 mouse, the laboratory species/strain most sensitive to abamectin toxicity. In addition, the degradates from photolysis of thin films were negative in the microbial mutogenesis assay (Crouch et al. 1992). The specific objectives of the present study were; (1) to estimate the percentage of mitotic abnormalities in male mice bone marrow cells induced by abamectin and its degradates, (2) to measure the amount of spermhead abnormalities in male mice caused by abamectin. Materials and methods Chemical Commercially available sample of abamectin, as 'Avermectin B1', 1.8% wt/vol emulsifiable concentrate, produced by Merck, Sharp & Dohme, Three Bridge, N. J., USA., was used in the bioassays. The chemical structure is a mixture of homologous products with a minimum of 80% B1a and a maximum of 20% B1b components. The B1b component differs chemically from the B1a component by only 1 methylene (CH2) unit at the 26-carbon position; the ethyl group (C2H5) is a methyl group in the B1b form. The compound's dilutions were made in distilled water.