Abstract
1. An EEL mains-operated colorimeter has been calibrated to obtain rapid estimates of the concentration of spermatozoa in bull semen.2. A general linear calibration relationship has been calculated givingh, the estimated concentration of spermatozoa per ml. of the original semen, as wheredis the dilution factor andPdis the absorptiometer reading.3. Under the circumstances of the experiments described, the accuracy of estimates from the above relation was about 1%, and it is considered that the instrument may be effectively used in semen determinations for artificial insemination and in most cases where a rapid determination is required.4. Attention is drawn to the desirability of using standards and occasional check calibrations, and to the possibility that in routine use results may be slightly less accurate than those given above, which were obtained with laboratory determinations.