Differences in the slopes of dose-response curves measuring pregnancy specific beta 1-glycoprotein (SP1) by means of radioimmunoassay.

Variants of pregnancy specific beta 1-glycoprotein have been described previously. These variants seem to cause artificially low levels when measured by radioimmunoassay. We demonstrate that sera with indistinct precipitates in electroimmunoassay give less steep dose-response curves in radioimmunoassay than do sera with well defined precipitates. Until parallel dose-response curves are demonstrated for all variants, previously published data must be treated with reserve.