Section 408(d)(3)(A)(ii) of the tax code regarding rollovers to an eligible retirement plan states:
(ii)the entire amount received (including money and any other property) is paid into an eligible retirement plan for the benefit of such individual not later than the 60th day after the date on which the payment or distribution is received, except that the maximum amount which may be paid into such plan may not exceed the portion of the amount received which is includible in gross income (determined without regard to this paragraph).
It's this statutory limitation on rolling over to the eligible retirement plan only the pre-tax portion of the distribution that creates the opportunity to separate the pre-tax portion of your traditional IRAs from the after-tax portion of your traditional IRAs. Note that if you have more than one traditional IRA, all of your traditional IRAs must be treated in aggregate for this purpose.
IRS Pub 590-A refers to this section of the tax code as follows:
Tax treatment of a rollover from a traditional IRA to an eligible retirement plan other than an IRA. Ordinarily, when you have basis in your IRAs, any distribution is considered to include both nontaxable and taxable amounts. Without a special rule, the nontaxable portion of such a distribution couldn’t be rolled over. However, a special rule treats a distribution you roll over into an eligible retirement plan as including only otherwise taxable amounts if the amount you either leave in your IRAs or don’t roll over is at least equal to your basis. The effect of this special rule is to make the amount in your traditional IRAs that you can roll over to an eligible retirement plan as large as possible
Before attempting such a rollover you would want to confirm with your employer plan that they will accept the rollover.
