GaAs layers grown by the molecular beam epitaxy (MBE) method at low temperatures (200-degrees-C) and also MBE samples grown at 300-degrees-C highly doped with Be or Si show high concentrations of As antisite-related defects in the optical absorption and in the magnetic circular dichroism of the absorption (MCDA). With optical detection of EPR it is shown that these antisite-related defects have properties similar to those of the EL2 defects except for the EL2 bleaching characteristics and the so called zero phonon line. Their spin-lattice relaxation time is strongly reduced compared with that of EL2+. These differences may be related to their high concentrations, which are of the order of approximately 10(19)-10(20) cm-3. In MBE samples grown at higher temperatures (325-degrees-C, 400-degrees-C) a new As antisite-related defect was detected with a reduced As-75 hyperfine splitting as compared with that of EL2+. This new As antisite-related defect has properties very similar to those of another As antisite-related defect previously detected in horizontal Bridgman n-type GaAs. A comparison of four different As antisite-related defects with similar reduced As-75 hyperfine splittings is presented.