From publisher blurb:
Microlite81 is based on the Basic and Expert boxed sets of the world's most popular fantasy roleplaying game published in 1981, often referred to as B/X. These rules are not intended to be a clone of the B/X rules, but rather a conversion of them to a rules-lite D20-based system that encourages old-school play without strictly old-school rules. These rules assume that the GM understands the basic concepts of roleplaying games, but provides information for both the GM and the players on the various “old school” styles of play. Microlite81 games can easily use adventures and material from B/X and other early editions of the world’s most popular tabletop fantasy roleplaying game or modern clones.
This is the standard version of Microlite81. The rules are about 14 pages, including detailed class descriptions for clerics, fighters, magic-users, thieves, dwarves, elves, and halflings. The rules cover combat, adventuring, building strongholds, magical research and more. Optional rules are included to make the game less like its Microlite20 base and more like the original B/X. Spell, monster, and treasure descriptions are also included.