Individual P elements that were genetically isolated from wild-type strains were tested for their abilities to repress two aspects of hybrid dysgenesis: gonadal dysgenesis and mutability of a double-P element-insertion allele of the singed locus (snw). These elements were also characterized by Southern blotting, polymerase chain reaction amplification and DNA sequencing. Three of the elements were 1.1-kb KP elements, one was a 1.2-kb element called D50, and one was a 0.5-kb element called SP. These three types of elements could encode polypeptides of 207, 204, and 14 amino acids, respectively. Gonadal dysgenesis was repressed by two of the KP elements (denoted KP(1) and KP(6)) and by SP, but not by the third KP element (KP(D)), nor by D50. Repression of gonadal dysgenesis was mediated by a maternal effect, or by a combination of zygotic and maternal effects generated by the P elements themselves. The mutability of snw was repressed by the KP(1) and KP(6) elements, by D50 and by SP, but not by KP(D); however, the SP element repressed snw mutability only when the transposase came from complete P elements and the D50 element repressed it only when the transposase came from the modified P element known as delta 2-3. In all cases, repression of snw mutability appeared to be mediated by a zygotic effect of the isolated P element. Each of the isolated elements was also tested for its ability to suppress the phenotype of a P-insertion mutation of the vestigial locus (vg21-3). D50 was a moderate suppressor whereas SP and the three KP elements had little or no effect. These results indicate that each isolated P element had its own profile of repression and suppression abilities. It is suggested that these abilities may be mediated by P-encoded polypeptides or by antisense P RNAs initiated from external genomic promoters.