A comparative study of 2 male recombination factors (31.1 MRF and 23.5 MRF) isolated from the same Southern Greek natural population, revealed specific differences in their activities. The 23.5 MRF induces female sterility due to atrophic ovaries at a wide range of temperatures while 31.1 MRF does so only at high temperatures. The gross morphology of the atrophic ovaries was the same and unilaterally affected pairs were found in the F1 of crosses with both 23.5 and 31.1 MRF. Furthermore, 23.5 MRF induces lower frequencies of abnormal anaphases I and II than 31.1 MRF, higher frequencies of double crossovers resulting from deficiencies or duplications, large clusters of recombinants, suggesting premeiotic origin and cases where 1 of the non-recombinant phenotypes was not produced. Such cases have never been observed with 31.1 MRF. The cytoplasm of the Cy L4/Pm strain that suppresses 31.1 MRF does not affect the activities of 23.5 MRF. Hypotheses to explain the different behavior of the 2 factors are presented and discussed.