Rhenium and osmium concentrations and osmium isotopic ratios of group IIA, IIIA, IVA, and IVB iron meteorites were determined by negative thermal ionization mass spectrometry with modified digestion and equilibration techniques. Precise isochrons are defined for all four groups. An absolute age of 4558 million years is assumed for group IIIA irons, leading to closure ages of 4537 +/- 8, 4456 +/- 25, and 4527 +/- 29 million years for the group IIA, IVA, and IVB irons, respectively. The initial osmium-187/osmium-188 ratios of the IIA, IIIA, and IVA groups, as a function of crystallization age, suggest that the rhenium/osmium ratio of the parental materials to these asteroidal cores was similar to that of ordinary chondrites, Data for IVB irons, in contrast, indicate a different mode of formation and derivation from a distinctly nonchondritic osmium reservoir.