Cardinals 1B Mark McGwire hits his 400th career home run in a 9 – 2 loss to the Mets. He is the 27th player to reach 400, and does so in fewer at bats than anyone in history, 4,726. Babe Ruth had taken 127 more at-bats, having held the old record.

On May 8, 1998 — Cardinals 1B Mark McGwire hits his 400th career home run in a 9 – 2 loss to the Mets. He is the 27th player to reach 400, and does so in fewer at bats than anyone in history, 4,726. Babe Ruth had taken 127 more at-bats, having held the old record.

Source:
Baseball Reference May 8