Summary. Friesian cows were inseminated with semen mixtures containing equal numbers of spermatozoa from a Friesian and a Hereford bull. The five bulls of each breed gave twenty-five possible combinations. The paternity of calves was established by inspection of colour and conformation. Heterospermic indices were calculated to express the relative ability of sires to father offspring after mixed insemination. There were significant differences between the heterospermic indices of bulls, the maximum observed difference being twenty-one-fold. The indices were consistent over two series. The homospermic index was defined as the 16-week non-return rate after normal single first inseminations. The heterospermic index established differences between bulls more efficiently than the homospermic index; one estimate showed that the heterospermic method needed less than 1/170th the number of inseminations required by the homospermic method. The homospermic index was predictable from the heterospermic index, the regression coefficient having a significance level of 0\m=.\05> P > 0\m=.\025.The initial spermatozoan concentration of a bull's semen (before dilution) was highly correlated with the heterospermic index. Measures of semen quality based on the morphology and staining affinity of spermatozoa predicted heterospermic and homospermic indices non-significantly but in the right direction.

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A total of 1800 eggs was examined by four experienced inspectors who estimated the Haugh unit value of each egg by candling. The eggs were of two different ages and were presented in random order to each inspector. On subsequent breaking out of the eggs the Haugh units values were measured, together with the shell thickness and yolk colour. Relations between the inspectors' estimated Haugh unit values, the measured values, and the other factors were then examined statistically. It was found that the age of egg affected the inspectors estimates' errors (the differences between their estimates and the measured values), as, to lesser extents, did the yolk colour and the shell thickness.While consistent patterns appeared in the " errors " of each inspector, their estimates of Haugh units values had low correlations (generally about 0-5) with the measured values, and similar correlations with each other. Thus it appeared that while they were trying to evaluate quality by different criteria from the measured values, they were also to some extent inconsistent with each other in their judgements.