An early lunch had been taken at Old Trafford, before the match was called off after an inspection at 12:25 BST.
Lancashire had trailed by 131 runs with one first-innings wicket left, but both sides take 10 points from the game.
Somerset remain fourth in the table, with Lancashire eight points behind them in fifth place.
Somerset's County Championship match at Lancashire ended in a draw after rain caused the fourth day's play to be cancelled without a ball being bowled.