Although a given foreign breed may offer worthwhile genetic improvement, the cost of importing purebred stock makes its evaluation difficult. An experiment was performed in which the breeds compared were represented instead by semen from boars standing at national artificial insemination stations in their native countries. Contemporary groups of Large White × Norwegian Landrace females were inseminated, some with British Large White (LW) semen, and some with semen from Canadian Yorkshire (CY), Danish Landrace (DL), Norwegian Landrace (NL), or United States (American) Duroc (AD) or Yorkshire (AY). One pair (male castrate and gilt) from each litter was performance tested from 27 to 82 kg live weight fed ad libitum (AL), another pairwas tested over the same weight range but fed to a time-based scale (TS). The number of LW, CY, DL, AD, AY and NL sires used was 51, 12, 21, 7, 6 and 23 respectively; a total of 401 litterswas evaluated. The differences between the progeny of LW sires and those of CY, DL, AD, AY and NL were, respectively: in daily live-weight gain (g), AL, –30, 40, –80, –50, –40 (maximum s.E. 16) with no significant differences for TS; in food conversion ratio, AL,0·02, – 0·13, 0·03, –0·05, 0·00 and TS, 000, – 0·14, 0·05, –0·03, –0·06 (maximum S.E. 0·049 for both feeding regimens); in percentage of lean by weight in the rumpback joint, AL, –2·1, –1·9, –1·8, 2·3, –0·7 and TS, – 0·1, –0·9, –1·9, 3·0, – 1·4 (maximum s.E. 1·28 for both); in overall economic worth (pence per pig sired), AL, – 150, 50, – 170, 110, – 40 and TS, – 10, 100, – 200, 190, 30 (maximum s.E. 83 for both). The U.S. Yorkshire and Danish Landrace seem unlikely to be of use commercially in the U.K., while the most promising breeds, U.S. Duroc and Canadian Yorkshire, have not yet been evaluated with sufficient precision to reduce the risk involved in their use to an acceptable level.